Not sure I follow this. Salesforce bought Heroku over a decade ago and Heroku is still improving in its areas of expertise, with no signs of changing that.
If anything, Salesforce has been moving more of its core developer experiences over to the Heroku model rather than trying to make Heroku more Salesforce-y.
After a short exposure to SFMC I can see why someone would be concerned about the impact they would have on Heroku, but it is good to hear there are reasons for optimism.
