Skype as a calling tool works fine and does the job - you hear each other, you see video, you discuss work stuff. If calls are going smoothly and nobody's complaining about connection quality, then technically it's not outdated at all. The thing is, the interface itself looks a bit dated compared to modern solutions, but that's more about visual perception.
The real issue to tackle isn't really Skype itself, but what's convenient for your entire team. Teams or Zoom really do have better-organized chat management, document handling, integration with other services. For example, if you're discussing a project, in Teams it's easier to keep files right in the team channel, share meeting recordings. That saves time hunting for information. But if your company's small and everyone just hops on calls without all the bells and whistles - maybe switching just isn't necessary.
If your colleagues keep bringing this up, you could frame a transition not as criticism of what you're using now, but as an improvement. Say something like: hey, I noticed modern platforms combine calling and teamwork in one place, maybe that'd make our lives easier with documents and keeping track of discussions. Show your manager a specific case where this would've simplified things - then it sounds like a solid argument, not just "we're behind the times."