How hard can it possibly be to replicate? Zoom walked into a market packed with established players and now they own the whole thing. That suggests the barriers to entry aren't so great.
I think the "high-quality video conferencing" is the "hard to replicate" portion. I've used a lot of different conferencing platform and consistently zoom has been: Relatively easy to use, high quality, reliable.
Those last two portions are super critical. No one wants to spend the first 10 minutes of their meeting fighting with their conferencing technology. Also no one wants to sound like robots.
With COVID19 and a ton more people working remote, will there be more competition coming into the space? Sure. But it will take them a lot of work, I'd think, to at least meet Zoom's quality and reliability... and do it either so much better or for so much cheaper to warrant change.
