Is there something about BTC that would prevent people from using other cryptocurrencies simultaneously?
No, you can. And some do.
Litecoin is a copy of Bitcoin with only a few changes and came out after Bitcoin, it doesn't offer any big advantage.
In theory Litecoins could be just as useful as Bitcoins, but there are only few reasons to migrate to them.
Not true. The dollar value right now makes me want to buy LTC more than BTC.
Do you realize that those numbers are quite arbitrary?
You can buy 100000000 of some cryptocoins for a single dollar, but there are also 1000000000000000x more of them aviable.
Also cryptocoins become more useful when more people use&accept them.
I think you hit the nail right on the head. A cryptocurrency can only succeed, if there is a limited number of them- ideally, one. How could BTC stop all other cryptocurrencies from achieving any traction?
Unless the others offer a very big advantage, they will not outdo Bitcoin.
They might be used for other niches though.
Maybe if a nation would put up it's own cryptocurrency it could gain enough traction to compete.
It's not impossible that a good competing cryptocurrency will emerge, but there isn't one at the moment.