Doesn't have to be a Mac only app. But be considerate to your users and use the best available SDKs for each platform, not one size fits all.
But what's your game plan under other circumstances? Do nothing?
Just because an enterprise has funding doesn't mean it's wealthy in other, more scarce things like organizational capital. In fact, I wouldn't be surprised if all the client developers at Slack would love to split off into dedicated platform teams and do a rewrite. We as developers love that shit.
But you should try to understand why these enterprises make these trade-offs and why they are still in these positions in spite of how much disposable money you think they have.