The Europeans didn't arrive for nearly a century after the treasure fleet,so there's plenty of time for China to entrench its' hold over the region.Besides that,the Europeans that arrived were too small in size to fight a maritime China effectively.Just look at the seventeenth century where Chinese maritime power has clearly regressed for example,the Chinese were still able to defeat the Europeans because the Europeans were simply out-manned and outgunned.
The treasure fleets finished in 1435. The huge costs of the fleets was enormous and China had no ability to continue supporting them over long period of time as threats from north and west demanded greater Chinese support and with low taxation of Chinese government there was not enough $ to support all enterprises. These treasure fleets provided little to no economic value to China and were very expensive with not only the cost of ships and men but also the cost of the gifts and bribes were given to other countries.
For the Chinese to of increased its power and influence in the region would of required one of two things military conquest or trade and settlement. Neither of which could of been maintained with the Chinese taxation policies at time but more importantly Chinese attitudes towards maritime trade and the Chinese who pursued it.
Any expansion into the region would of been at the expanse of other powers and interests. So Europeans while few at beginning would of aligned themselves with the powers who were jealous and upset at Chinese in region.
Now let’s look at timeline.
Chinese treasure fleet voyages 1421-1435.
At this point some decision needs to be made how to proceed and how to find the maritime expansion. The Chinese would of required to either build more ships to launch invasions or to restructure their naval forces to expand their trade network and protect trade. This would of taken years if not decades and at all times at the whim of next emperor who may be of opinion the cost not worth it. Threats from north or west would of taxed chinese resources and more than likely diverted money to those regions.
Realistic if a continued strategy to expand Chinese influence was continued we could of seen substancial Chinese power in the later part of the 15th century. Post 1475 at earliest.
Yes the Portuguese with their supperior ships would of found a very different south east Asia but still able to establish themselves in Ceylon and India. They would of either traded with Chinese and their subject nations of aligned themselves with other nations who opposed the Chinese. That would take us to the end of the 16th century.
Now enter the Dutch and English who hungry for the riches of the east would of been able to array more power and forces against the Chinese.
All this time China has to continue investing huge amounts of money into the subject nations, naval fleet. Hoping that revenue from trade would of payed a large or all part of those costs. All of this was only possible with a huge physiological change in attitude by Chinese government and those in power towards Chinese merchants and sailors.
A huge task and one that one slip up and misguided decision bring about its collapse.
Look at the Portuguese, they lost to the Dutch due to over extensions, lack of personnel. Unfortunately they did not have access to a another group of people like the Norwegian sailors who made up a huge portion of VOC sailors. The Portuguese also suffered the huge loss of majority of its leadership including king in 1536 which led to disastrous Iberian union and destruction of huge portion of Portuguese fleet in the ill fated attempted Spanish invasion of England.
The Dutch also were able to sail to the Indian and East Indies because the Dutch spies stole the copies of the Portuguese maritime charts so they were not sailing there blindly.
So huge obstacles in Chinese ways both internally and externally.