"Pre-2010"? Heroku only launched in April 2009. So I would assume the vast majority of current customers are not actually from a "pre-2010 era". Heroku didn't even have a postgres add-on until November 2010 -- the salesforce purchase went through in Dec 2010. In fact, the heroku "golden years" were mostly during the early salesforce period. (Heroku put matz, creator of ruby, on staff in July 2011; there was no Heroku API until 2014!). (source for timeline:
https://www.heroku.com/about)
Heroku is still quite a bit of value-added over "AWS with some basic CI integration", people chose it and still choose it because it requires a lot lot lot less in-house expertise and management hours than AWS for most kinds of standard apps. (I'm not totally sure what AWS services/architecutres you are thinking of when you say that; but I'll say: for pretty much all of them.)
(Heroku def has more competition in that space than it did 10 years ago, opinions differ on the relative merits)
AWS of course already existed when Heroku was launched, so if you do consider them just "AWS with some CI integration" then I guess they would have been from the start? What would have made that true now if it wasn't then?
I think Salesforce thought it would somehow be more "synergistic" with their other offerings than it has turned out to be. That they'd get Salesforce customers on heroku when they needed something beyond the "no-code" tools Salesforce already provided, or that they'd do better at converting heroku customers to salesforce customers than they have been. It does seem to be true that salesforce stopped really investing in new heroku features or improvements some years ago, and seem to be looking to minimize costs while continuing to collect revenue, I agree. (Sometimes I'm not even sure how much they care about continuing to collect revenue...)