That's really out of character for Powell, he never really acted in the typical political way, worrying constantly about his numbers. He knew that the speech would tarnish his chances for a cabinet position, at least one of respect, and would lead to people harassing him. He wouldn't care or would be so confident that people felt the same way that he would just give the speech anyway. This is, after all, the guy who endorsed the opposing party in 1974 and openly blamed riots on people he found foreign. No matter what you think of what he said, he was, without a doubt, unflinching in his beliefs, and would not hesitate to speak his mind.
But, as an exercise in thought, if he didn't give the speech or the Morecambe Budget speech, we could potentially not see Thatcherism, at least not the way it played out, and the Conservatives may not have won the 1970 election, or at the very least, the entire 1974 election would have been totally different. Powell was instrumental in introducing increasingly right-wing policies into the mainstream and providing a challenge to previously widely supported one-nation conservatism, and the whole post-war consensus as a whole. He doesn't really have a comparison in modern-day politics... he was loved by many and hated by many simultaneously because of his controversial beliefs. Probably only Nigel Farage or Bernie Sanders in the US could come close to him, and even then they are much less controversial then he was. It's like removing Farage from UK politics, without him, the entire post-2010 UK would be unrecognizable, Brexit wouldn't happen and David Cameron would probably still be in No.10, the same thing with Sanders, without him, the Democratic Party would probably still be in the mold of Clinton and Obama, third-way centrism instead of progressivism.
Let's say that without the speech, Heath would still win the 1970 election, and narrowly win in 1974 due to a general lack of division within the Tories. He loses in 1979, due to stagflation and the horrible late-70s that would tear any leader to shreds, probably to Michael Foot, who would be elected leader because he's popular with unions and radical. He would be the Thatcher of this world, the person who breaks politics because he is so far left. He would not, in any way, help this. He would nationalize more industries, attempt to abolish the House of Lords (failing to do so because of Blue Labour MPs who wouldn't support it, at least not yet), and he would be the cause of another major change. A relatively small breakaway faction would cause him to lose his majority, and in 1982, there would be another general election. This faction, call them whatever you would like (preferably SDP), would ally with a Liberal Party that would, justifiably, call for an end to the current system, which became a choice between a communist and an ideologically timid one-nation conservative, probably Whitelaw. Labour would lose in a landslide, especially after they surrender the Falklands, giving them the image as the party that wants to surrender the national identity. This leads to a hung parliament and mayhem, as Foot is thrown out of office as Leader in favor of Kinnock, and the alliance, which is now in opposition with Labour close behind. The Tories are the only people who could remotely be seen as authoritative, and they attempt to find a working coalition with the Alliance, who are resistant to it. The Tories form a minority government, and immediately call another election, increasing their seat share at the further expense of Labour, but not securing a majority. May I add that this will most likely change the US as well. Without Thatcher, Reagan wouldn't have as much diplomatic or political capital, especially if it's Michael Foot in office.
From there, it's difficult to see what will happen... but needless to say, Powell changed politics, and the world, no matter what you think about what he said. Without the Rivers of Blood Speech, everything is different, down to the way we talk about issues. It's the epoch for our current political discourse, and a monumental event in modern political history.