It should be noted that once the Network effect [0] takes place, it becomes less about freedom to build your own software and more about the cost (including - maybe especially - intangible) and viability of doing so.
If the problem is "this user is unsatisfied with this product', the solution is not necessarily to tell them to build their own [1]. We, as people, are allowed to criticize (hopefully constructively) products that have flaws in them even while using said software.
I'm not so sure the network effect is significant here.
Slack is designed as a space for team communication, so it seems like a situation where the network effect is at it's least powerful. After all, teams are about the smallest size imaginable. Any alternative is also going to be on the Internet, be it another product, IRC, or a mailing list. The only inertia you have to overcome is the inertia to create an account.
Second of all, the value of peers on a team network is not equal. The team leadership and top contributors are the most valuable members of the network. If they decide to jump ship to another product, your choice is to either follow suit or essentially exclude yourself from the team.
Would you seriously not work someplace or not contribute to a product because it meant you couldn't use Slack and had to use Hangouts or IRC? I'll agree that Slack's product is better and -- baring complaints common to many Electron apps -- relatively without fault, but requiring it seems a bit excessive.
If the problem is "this user is unsatisfied with this product', the solution is not necessarily to tell them to build their own [1]. We, as people, are allowed to criticize (hopefully constructively) products that have flaws in them even while using said software.
[0] https://en.wikipedia.org/wiki/Network_effect
[1] https://en.wikipedia.org/wiki/False_dilemma