From my point of view, as a mere customer of their mass-market product (a.k.a. cedar) with some insight into operation history, I think expensive or at least detailed investment in refurbishing or adjusting the mass-market product became, for whatever reason, difficult to (internally) justify expense for.
For example: the mechanisms that make "heroku run" work could have really used a new version that could let me change my window size gracefully, and maybe streamlining some authentication, by using SSH. As far as I know, it's closely related to the first version of that feature ever written, which somewhat naively ties the client machine's tty to a remote tty via TLS connection. This is fine for getting cedar off the ground in the first release, but at some later year (decade?), it seems like it should have gotten re-do so, finally, I could change my terminal emulator window size.
Ditto the holding of extremely powerful API credentials in the plaintext file `~/.netrc`, further exacerbated by a fairly weak authorization system for Cedar. There's still a place for .netrc (e.g. so curl can pick up the credentials automatically), but at some point, use of the keyrings on various operating systems (at least) and U2F hardware attestation should have shown up.
Passing along cost reductions in inputs (e.g. AWS servers) is another long-standing example.
Somewhat related to that, IPv6 is a lot more common than it used to be. So are VPCs or similar products on other platforms. Putting all that behind a "let's have a call" process -- and cost structure -- for "Private Spaces" is not in step with the times in 2022. And probably not 2018, neither.
As a counter-example, it does seem the web/dashboard login not only grew U2F, but Macintosh TouchID support. Whoever got this in: well done. But it's a noteworthy exception. And I kind of wonder if it has anything to do with the web authentication system being more trouble than it's worth to segment between Cedar and the Private Spaces product.
There's always a temptation to put new features into an upmarket product to increase the gap in segmentation, but realizing that at least some features are closer to adaptations to raising or at least changing standards of adequacy in industry does not come naturally. I don't think this is the only problem, but it seems like a problem to me. As it comes to projects that turn on minute details, such as improving "heroku run" in the presence of terminal emulator resizing, it probably says something about the bureaucracy's inability to grapple with details of that kind. That was already present when I left.