"open themselves to have more developers supporting theirs?" -- Thats exactly what is missing from Google/Microsoft.
Apple is already supporting Swift on other platforms, however, developer experience is not as good as on macOS and I don't think that can/will be solved by Apple.
It is exactly where other players missing out, by making their dev experience as good or better, they could help developers write apps for their platforms.
> Apple is already supporting Swift on other platforms, however, developer experience is not as good as on macOS and I don't think that can/will be solved by Apple
Yes it can. It's just Apple can't be bothered - most of the work to do that doesn't benefit Apple - it does benefit the Swift ecosystem and community of devs, but not Apple - the venn diagram of Apple vs the Swift dev community overlaps but doesn't completely close.
Swift came out in 2014 - Windows support for the compiler came out in 2020. That's it. They haven't bothered to upgrade the tooling support in VSCode or any other app because most VSCode users aren't Apple devs. The ball is literally in Apple's court - no one elses.
.net and kotlin work on Mac. Why would MS or Google subjugate themselves to an ecosystem driven by Apple?
Plus, honestly, moving from swift to kotlin or even dotnet is marginal. The problem is getting into the qwirks of the underlying frameworks like swiftui, the ios sdk, the android sdk, and the conventions of these platforms. Language itself is relatively marginal in that. So really I don't see the value in what you're claiming.
