People tend to forget what a blessing Flash was when it first arrived on the web, how horrible other plugins were, like the awful Java mess. Everytime a Java-applet tried to run, you could only pray it wouldn't crash your browser and take your whole system down. Or the terrible experience called RealPlayer! Sure, Flash has a bad security reputation, for good reason, but it made the web more beautiful, interactive,fun and usable at the beginning of the new century. All you had to do was click on "Skip website intro".

I highly doubt Flash did anything to significantly change this. After images and audio, the web was screaming for video. If it weren't for flash, perhaps we'd instead have links that opened up native video players, or whatever, and we'd be better off for it.

In short: YouTube would have been a success even in the alternate universe where Flash never existed.

Steam engines were an amazing innovation as well. They brought countless improvements to society, and were a precursor to true internal combustion. However, no one relies on them in normal daily life anymore as there are much better alternatives. We can move on from a technology and still honor what it brought to the world.