Well, that's what makes cryptocurrency a worthwhile market for speculation. Many folks look to the "market cap ranking" to see what's hot in the space, and going down the list and positioning yourself in a rank 400 coin - a good one that shows promise as a tech platform, a more efficient coin, a useful financial instrument or whatever - is one of the most straightforward ways to see 1000% returns, because even if it doesn't make it all the way up to the top 10, its valuation can grow faster than the market as a whole. (It can also get hit with huge drawdowns when the bear market hits. It's not all roses...)

While it is the case that most things get priced relative to BTC, and this does have a network effect since you end up exiting to BTC and thus reenforce its price, it's increasingly common to see trading pairs for ETH, LTC, USDC/USDT. There is interest in getting more liquidity into the market and that's a large part of "DeFi". It's not that BTC is self-perpetuating lock-in, it's that they're all growing but everything else is running several years behind, so on a day to day basis BTC still holds relatively more of the market.

Bitcoin is leading with a 50% market share. So big but not really a monopoly.

The question what will happen when Bitcoin flops. Will people trust other crypto? It looks like for the crypto train to keep on chugging, bitcoin needs to be sustained.

Right now it seems to be the main driver indeed. If the 'store of value' Bitcoin fails, I guess in the short term, all crypto's will crash. And then the question will be if one has a decent enough value proposition to rise up again. Although a lot of people are opinionated on this last part, the fact remains that none of us really knows.

Another option is of course that one crypto would just overtake Bitcoin. If I remember correctly this almost happend with Ethereum and was called the flippening. This scenario would be better for crypto of course than a real Bitcoin crash.