I can definitely understand this decision. We used to use Slack in my previous company which was a digital agency and being a client manager it was incredibly hard for me to keep track of the numerous chat messages i depended on in Slack but at the same time keeping up with the clients which were always, no matter how hard we tried - active only on email.
Altough Slack is a great concept i think in general the biggest problem of proprietary platforms is that usually the original task always starts somewhere else. Slack can work great as an end-point solution, where people don't need any other tools to exchange business content. If you think about it - most of the service driven business across the planet initially starts with a phone call or an email. After it successfully comes down the pipeline to a done deal - you kick it off with a meeting or something, you decide about the desired channel of communication and you can kick off the project.
Sure you set up some rules - like separate channels for unimportant topics or maybe you put separate tasks in a different tool etc.
If everyone sticks to the plan (including the client), than Slack works this way.
But not everyone sticks to the plan. It's because phone calls happen or emails happen in between, where you have to move discussions from where it started back to Slack and this was the whole problem for us. We were working a lot on improving our client's rate of happiness - and there was a clear correlation between them being satisfied with the work that was done and us adopting to their tech stack. Not the other way around. Everything that had to be moved to Slack, so it could be considered as 'a project'and people would keep track off - eventually was missing some sort of information that happened on email sometime during the process. And for us, that didn't help with improving our services.
So from my experiences, when working with services, the ideal tech stack situation for companies comes down to how companies work with clients. If clients can be adaptive to the core communication platform, than you'll find enough integrations to make the best work flows to help you optimise how the business work. But if you're working with clients that usually have their own preferred tech stack or involve more people - that the chances of Slack interrupting your workflows are much higher.