Sure they have a huge market share. That's simply from being the first video-sharing platform and being from Google.
There is nothing unique or irreplaceable about them and plenty alternatives do exactly the same thing now.
Only better without ads.
Problem is there are too many and divide the small leftover market share between them.
When/if that's fixed people will not look back.
'normal' people choose VLC now and other FOSS.
YT is not their favorite sportsteam, they have no loyalty to them.
On the contrary. If someone insists on playing YT music for friends and it's interrupted by an ad, everyone's annoyed.
And content is shared on as many platforms as they can since it's free and they want as much viewers as possible.