Not surprisingly, most of the answers on HN are from an engineering perspective.
When I started selling the first products I designed and manufactured --decades ago-- I absolutely sucked at it. I was selling like an engineer thinks. Which means I was a horrible sales person. It wasn't until one of my friends, who happened to be one of my resellers, took me under his wing and taught me sales that I understood the process. It took somewhere around six to eight months for me to "see the light". Towards the end I could sell our products almost without saying a word about them. You are dealing with people, not robots. Everything you care about as an engineer is usually of no interest whatsoever to the buyer (that can be the case even for highly technical products).
I think the answer is far simpler than the obvious go-to's in this case (established, network, google, infrastructure, technical blah, blah, etc.). Sure, those are factors, but this is about sales and sales is about psychology.
Simple question:
What would it take to sell anyone on Y and have them stop stop using X?
Let's say X is a brand of forks and knives and Y is a different brand. Furthermore, assume they are free. Cost of the switch is exactly $0. Effort is also zero. You say "I want to switch to Y" and they magically appear in your kitchen and replace X. As easy as can be.
Well, Y has to give you a reason strong enough to compel you to make the change. Call it value, if you will. The mythical "differentiation" with, perhaps, more attached to it than just being different.
A few weeks ago I saw metal cutlery that was black. It looked beautiful. Same stainless steel material usually used for cutlery everyone if familiar with. Except, in this case, it was blackened, likely using a chemical process. If that was Y, it could inspire some to make the switch, just to be different. Maybe. More likely than that, they'll get Y and keep using X.
Put a different way, if X works well and does everything you want it to do, there are very few reasons to change to Y. The company behind X might have to do something horrible to suddenly inspire mass exodus.
I believe this is the case with YouTube/Google. What they do, they do well. The user experience is excellent. Yes, we all know about the ridiculous no-customer-service account suspensions where you lose all of your Google app access, etc. However, this is not the average user experience. In fact, I would venture to say that this is well outside two standard deviations from the mean, ridiculously outside of that. The area under the curve is deeply dominated by users who are satisfied to the extent that the thought of going elsewhere never really crosses their minds. When a user/customer/client can't be compelled to even think about Y when using X, the probability of them considering a switch is as close to zero as can be.
The only way for someone to mount a solid effort against YouTube falls under two areas:
A- By force. Spend billions advertising and educating the audience and, over time, if the product is good and the message is solid, N% of YouTube users would migrate. This would require so much money it would easily meet the definition of insanity. The cost of acquisition isn't likely to ever justify the investment.
B- Cater to a deeply motivated audience to carve out a much smaller percentage of YouTube's audience. The most obvious audience I can identify in this moment in history is Trump's audience. Regardless of what the reader might thing of them, they have grievances with social media and YouTube that could be addressed by an alternative platform. The cost of acquisition, in this case, would be relatively low. In fact, I would not be surprised if most of the investment went into creating a solid infrastructure with only a modest marketing push to trigger network effects.
Other than option B --which would only capture an audience in the tens of millions, and possibly grow it to >100million over time--- I can't see anything that would inspire people to leave YouTube behind. And, even with B, the same audience would continue to use YouTube, because, well, everything is already there and the user experience is good. B takes advantage of deep motivation for change. Without that, it simply isn't going to happen.