NB. I might have confused MVC a little in my idea above.
But if the cores of different applications are interchangeable anyway (which they need to be, to make this model work), then why would multiple cores exist in the first place? It could just be reduced to a single one, and there'd be little incentive to start a new one if you can't deviate from the API anyway.
The closest real-world model to what you're describing, is an "implement once" public commons model; a shared commons of open-source / public domain libraries that implement a given responsibility or solution for a given problem, and that can then be used forever by everything, and that problem never needs solving again.
For that to work, the implementations in question need to be highly granular, so that using them is a no-brainer and you don't pull in additional complexity that you don't need. An "e-mail client", for example, would then effectively just be a UI, and all the business logic would be provided externally by a collection of loosely-coupled libraries.
Such a granular ecosystem, in turn, requires a dependency model where adding a dependency is (in and of itself) free and cannot cause dependency conflicts; otherwise people would start grouping complexity together again to reduce conflicts, as can be seen in the dependency ecosystems of many languages today.
The only ecosystem that is currently at a point where it can support the "implement once" model, is that of JS, due to everything being highly modular and interoperable. Rust is also moving in that direction, but its ecosystem is nowhere near mature enough yet.
Unfortunately, people are too busy complaining about meaningless dependency counts to understand what a highly-granular dependency model makes possible.
And worst case: MS change how things work and block this sort of thing happening again. You now have a bunch of angry customers with a hack that doesn't work.
I'm no lawyer, so I'd also check the legal implications of selling something that builds on top and modifies Microsofts application without their explicit consent.
Of course, the trade-offs of "sharecropping" on someone else's platform are always there, from a strategic point of view. MS can wake up one day and rewrite Outlook in Rust, forcing you to rewrite from scratch or making it impossible in practice. But it's a possibility much more remote, imho, than the chance of making significant money with yet another exotic mail client.
Sort of like mailgun but for consumer clients.
Btw, we kinda had something like that: Lotus Notes and MS Exchange, originally, were abstract document stores. In theory, anyone could have built custom views that behaved as you wished, while the platform took care of email fetching and synchronizing. Unfortunately, both Lotus (later IBM) and Microsoft saw those platforms as lock-in instruments in a push for vertical dominance, so they had no incentive but to push their own clients.
They are not dead, they are just out of your view.
Commercially, you see the occasional upstart, but inevitably they die after a few years. I used Airmail for a while but they just stagnated (they barely kept up with Gmail and struggled with Exchange) so I had to drop them.
The market for commercial desktop email clients is super-small: the intersection of the already-niche subgroups of people who will pay for email clients, people who will pay for desktop software, and people who will pay for already-commoditized software. The effort is simply not worth the small rewards, particularly compared to the impact one could have by building on existing commodity clients.
> MailMate is not the most widespread, the cheapest, or the greatest looking email client, but I also have no aspiration to MailMate ever being any of these. Instead, MailMate aspires to be the most powerful, the most flexible, the most efficient, the most standards compliant, and the most secure email client.
Instead, sell me a hack that can make Outlook look like that. You worry exclusively about the UI, Microsoft does the heavy lifting behind the scenes.
Best case: you make a ton of money by selling to actual businesses (rather than annoying geeks and other weirdos who dare use uncommon clients); slightly-worse case: you sell it to MS.