Slack is not in the same space as Basecamp, work tracking vs chat is a dramatically different use case. A more apt comparison would be Asana, but I'm not particularly worried about that either: there isn't really a one-size fits all approach for work tracking. I actually worry more about the future of Asana because they have those VC-fueled dreams and Facebook-cofounder-level expectations that mean anything less than a moonshot is a disappointment. With all those resources Asana can do a lot of things Basecamp can't, but there's no guarantee that can translate that all into the tremendous sustained growth they need to meet their goals.
I don't understand what is your point. The questions was "Is it desirable?" It's not - it's a tradeoff. More employees create more complexity, but it could be the case that's sine qua non to get create the product and capture the market.
Slack is IRC, Basecamp is a project management app, they really aren't even in the same space. Slack will never make Basecamp irrelevant, they're not playing the same game and aren't in the same space.
They're really no longer a product suite company, they renamed themselves Basecamp as that's their only real success and they've started selling off their other products.
