They should've open sourced it and invested heavily in security, stability and performance.
People blamed Apple back in the day for not including it on iOS but given that how many critical bugs have been found in the time since then you can't blame them.
Someone from Adobe once told me the main reason holding back open sourcing the player was licensing. Apparently the player consists of quite a bit of licensed technology that Adobe obviously couldn't open up, and some of these are so core to the player that refactoring it wouldn't really be feasible or not really worth doing. Rewriting it to not use these technologies or use them in such a way that they could be plugged in with alternatives would be too expensive and for little apparent business gain.
I paraphrase of course and I can't really vouch for the veracity of these claims, but I'm hoping someone with first hand information might be able to corroborate or deny. Sounds plausible to me.
I worked at Adobe in those days and AFAIK this is correct. The Flash player includes a bunch of codecs (mp3, on2, nellymoser, sparc, and I think others), and I believe there's a variety of other licensed code but I don't remember the details.
If they would have open sourced flash player I think we'd be in a much different place today. Not like they'd lose any ground on their actual profit center - the Flash animation tool and development environment, which is still seriously something fairly unique.
Maybe they can still reach that state if they invest serious effort in making their tool able to export HTML5.
I wish they'd just come up with Flash X (sub X for whatever), where the tooling was flash in a flash interface, and geared towards react in a flex-like (flashbuilder) interface. Where the output was a directory with assets that could run directly in a browser/iframe, and svg for vector assets.
A company like Adobe, constantly struggling with accusations of greed and carelessness, should consider this as a PR exercise and budget accordingly.
My guess is that they won't do it because there might be proprietary stuff they don't own, in the codebase. Macromedia wasn't that huge a company, I wouldn't be surprised to learn they embedded 3rd party components here or there. At that point, untangling legals might become very hard.
People blamed Apple back in the day for not including it on iOS but given that how many critical bugs have been found in the time since then you can't blame them.