They're a business so the question is not whether they technically have the cash available to do a thing, the question is whether doing that thing will yield the best return on investment compared to all the other things they could do.
Specifically, if Slack created new development teams to build native clients for Mac and Windows, would that effort eventually yield enough additional revenue to be worthwhile?
(Mind you, I don't know the answer to that question because I don't know Slack's financials, just pointing out that it's the real question governing whether they should use Electron.)