There used to be several 'universal' chat clients, for example Pidgin.
That doesn't mean it's easy or cheap. The moat is more in the installed base of data centers, edge networking, etc. US cloud providers undeniably have a bit of a head start there.
But the EU has a lot of domestic infrastructure as well. And the US outsourced a lot of things as well. E.g. mobile infrastructure and networking is now dominated by Chinese (Huawei) and European companies (Ericson, Nokia). Former telecom giants like Motorola seem to have faded away. Nokia actually owns Bell Labs currently. And of course a lot of the software involved is open source with a very international developer community. The hardware comes from Asia or in some cases Europe. ASML is Dutch, ARM is nominally still headquartered in the UK. Ownership of these companies is of course more complex.
For Slack and Teams, I'd say not even that is necessary. They're not meant as broadcast tools, even if they could be used as such, they're tools for… well, for teams. Either within a single company or B2B stuff. Given how powerful all the client devices are, the remaining work of the server to coordinate them all should, in 99% of cases, be so low that you can offload it to someone's bluetooth earbuds (as per recent story of Doom being ported and the conversions it led to about the typical modern embedded processor, and what we could do back when servers were that powerful).
It's not like every Slack/Teams instance is also running some clone of Google's Page Rank indexing of the entire internet locally.
The European Economic Area + UK also have a lot of telecoms and networking experience. If they have to pay for improvements to edge networking for a reliable replacement for Teams they could easily bring farm that work out to their telcos.
With enough political motivation barriers will be removed one way or another.
I do have to use Teams occasionally for work and bizarrely the web client in Firefox works far better than the native Linux Teams client. Not particularly difficult as the Linux Teams client wouldn't do anything except display a blank box (this was on Ubuntu).
I hadn't heard about PWAs (Progressive Web Apps) before, but it doesn't look like they're commonly used on Linux. At a first glance, they look a bit similar to ActiveX to me.
I disagree
The thing with Zoom, Meets, Teams, etc. is that these aren't that hard to replicate. There is not much of a technical moat. It doesn't take a very large startup to create your own version of that. And given what a basket case teams is, it's also not that hard to do much better. There have been plenty of alternatives over the years. Network effect is what drives the growth there, not technical quality.
So if the French want to use something else, all they have to do is pick something and they might get the network effect through mass adoption. That would work better if the whole of the EU does it of course. We'd still need a solution if we want to talk to people in the US. The reason why US drives the network effect traditionally is its trade relations. It's convenient for everyone to use the same tools and solutions.