It's actually not that terrible, but I believe it's nowhere near feature parity. Not to mention there is a galaxy of add-ons that still need to be rewritten from scratch to work on the web version, and a lot of people cannot live without them.

Excel is a classic case of 80% of users requiring 20% of all functionality, but that 20% being different for each user. Still, Microsoft made it very clear that the future is web-first.