Here's a TL as to how Portugal could retain its colonies until the present day.
After Salazar is incapacitated by a stroke in 1968, a hardliner comes to power in Portugal. Marcelo Caetano was a moderate who was unable to please the hardliners or the reformists at the time and it ultimately led to his undoing in 1974. Let's say it's someone like General Kaulza de Arriaga, who was far more rightwing but also was much more into realpolitik than Caetano.
Portugal was facing five guerilla movements in the Portuguese Guinea, Angola and Mozambique at the time. The PAIGCV in Portuguese Guinea was the most successful. They were in control of 3/4 of the territory by that time and were backed by the USSR. Arriaga favoured granting independence to Guinea-Bissau as the territory was small and had always been an economic burden on Portugal. The war made it more so. By 1973 there were over 30,000 Portuguese soldiers stationed there and the territory was draining about 1/3 of Portugal's defence budget. The casualty rate here for soldiers was also much higher than in Angola or Mozambique. Marcelo Caetano feared giving independence here as he feared it would have led to a "domino effect". However what ended up happening was that the officers stationed in Portuguese Guinea were among the most disattisfied and most exposed to Communist propaganda. General Spinola the military governor of Portuguese Guinea would lead the revolution in Portugal in 1974. So getting rid of Portuguese Guinea frees up money, men and material to concentrate on Angola and Mozambique.
Angola had 3 "liberation movements" who by 1972 were fighting against each other rather than against the Portuguese. The UPA/FNLA was based in neighbouring Zaire and had the backing of Mobuto Sese Seko. They drew their support from the Bakongo people however by 1974 they were fighting against the Soviet backed MPLA. The MPLA fought mainly in Eastern Angola from bases in Zambia. UNITA was the smallest group and had a secret cease-fire agreement with the Portuguese and were fighting against the MPLA. By 1974 the war was quite in Angola, so much so that 10,000 troops were transferred from Angola to Mozambique.
In Mozambique Chinese-backed FRELIMO had been fighting against the Portuguese using bases in Tanzania. Most of the fighting occurred in the remote Northeast district of Cabo Delgado where few Portuguese settlers lived. By 1971 they began trying to attack the Cabora Bassa damn which the Portuguese were building on the Zambezi. Though things had gotten worse after 1972, the war was not nearly as bad as in Portuguese Guinea and with the additional troops and material freed up for Mozambique, the stalemate could have been reversed.
So I argue that with Portuguese Guinea being set free in 1968 or 1969 the Portuguese revolution in 1974 could have been averted. Portuguese army could have focussed on Angola and Mozambique. Both territories were experiencing huge amounts of economic growth. In Angola this was especially spurred by the growing petroleum output from Cabinda. This led to huge increases in the number of Portuguese settlers in both Angola and Mozambique. In 1940 there were 72,000 whites in both territories, by 1960 that number had reached 270,000 and by 1974 there were 650,000 Portuguese settlers in both territories.
The Portuguese remaining in Angola and Mozambique would have had effects on neighbouring South Africa as well as Rhodesia. Without the Portuguese departure from Angola and Mozambique the South Africans would have never had to invade Angola to protect Southwest Africa (Namibia). Without bases in Angola, SWAPO would have never gotten far. Also this would not have focussed so much unwanted international attention on South Africa leading to the UN arms embargo in 1977. In Rhodesia the ZANU and ZAPU did not have much success until they had Mozambique as a base to operate out of so the white regime could have survived and propsered there despite sanctions.
The 1980s would have been very interesting. With Margaret Thatcher coming to power in 1979 one assumes that her government would have been predisposed to settling the Rhodesia crisis by 1981. Embarassed by the issue a compromise would have been agreed to where African parliamentary representation would be increased incrementally over the years and Britain would provide funds for "African advancement" helping to improve the living conditions of the African majority. Unrest in South Africa would have certainly been less since the ANC would not have neighbouring countries to use as bases of attack against the government. The government here would not have had to spend as much money on defence. Also should sanctions come into force (it's less likely that they'll be as severe) the Portuguese will be willing to supply the South Africans through Mozambique as they did for the Rhodesians. With Reagan coming to power in 1980 in the USA and the coldwar heating up in Africa the American government would most likely be more willing to aid a NATO ally in fighting USSR backed guerilla movements. The Soviets had been wanting to expand their influence in Africa at this time by backing various regimes in Ethiopia, Somalia, etc. The war in Angola would have been won in the 1970s and the Chinese had really stopped caring about African revolutionary and socialist movents by the late 1970s when they cozied up with the West.
After the 1990s with the collapse of the Soviet Union the guerilla movements have no financial backers left. The neighbouring African states of Zambia, Zaire and Tanzania can no longer play the coldwar rivalry to get aid and are poorer than ever. Malawi was always friendly to the three countries. Zambia will probably fall into their orbit after the death of Kenneth Kaunda. Tanzania is far too weak and poor and no longer has an idealist leader like Julius Nyrere. It would be interesting to see if the three regimes would back a side in the Congo civil war, maybe wanting to control the riches of the Katanga. South Africa, Rhodesia, Angola and Mozambique would be the most prosperous areas in Africa and the African countries are weaker than ever. The issue really is that colonialism and minority rule are seen negatively in a world where building democracy and human rights is seen as paramount. The Portuguese probably can escape sanctions since they aren't so much racist and will likely increase the number of Africans in the government (By 1973 Mozambique's provincial assembly was already majority non-white). One also has to remember that the Portuguese have more than enough oil in Angola to supply all three countries and still have enough left over. Rhodesia has Africans involved in government, but most international criticism will be against apartheid in South Africa.