I have friends who argue that Salesforce has neglected Heroku for a while so this shouldn't be much of a surprise to anyone now.
I haven't used Heroku too much myself.
Salesforce doesn't need a free tier to generate product leads. They have an army of sales people to push the product to customers that will write them big fat checks, then hand down edicts from the CTO's office requiring their internal devs use the technology.
Heroku has two core tenets - developer experience and upgrade to paid tier for production workloads.
As a result, at least the Heroku customers I know use the free tier for a good portion of their _non_ customer-facing production workloads - prototypes, staging, administrative functions, and so on.
This both increases their cost and makes budget planning a lot less stable. It means developers may become motivated to start coming up with workflows that target other environments where they would normally target Heroku, which like robs Heroku of a source for future revenue. Once you take on the devops work yourself, Heroku is no longer price competitive.
In other words, this new cost makes the unique value of Heroku look quite a bit more like a detriment. Thats rather unfortunate.
Heroku stock price is down roughly %50 from less than a year ago.
Death spiral? I don't know. But slashing costs is certainly not indicative of Heroku having a super awesome fun time.