As much as I think almost all altcoins are either trash or scams, bitcoin is not the only possible incarnation of a decentralized peer to peer currency. The idea might never die, but that does not mean that bitcoin is invulnerable to attack.
But, Bitcoin, after all,
is an idea. The fact that someone may catch up and gain the majority of the votes doesn't mean that he ruins the system. It's an idea which transforms human's greed to common good, which discourages you from attacking it, but rather helping it so you can earn the most.
There's harmony in this consensus and the only ones who can destroy it are the ones who make this beautiful melody.
There is no requirement to own bitcoin to attack it. A government which sees it as a threat to their fiat monetary system could be attempting to preserve their wealth by attacking bitcoin.
You may need no Bitcoins, but you need wealth. You can't plan to spend billions of dollars worth of ASICs and electricity
and retain their work until further notice (?), without damaging your own wealth. Besides, what could they do on a PoS crypto? Buy half of its units? They can't attack everything and that makes the 51% attack of Bitcoin meaningless.
No idea. I don't really follow altcoins very much. The only altcoin I hold is Monero, for obvious reasons given my feelings regarding privacy.
There are greater achievements in terms of technology than Bitcoin. Sure thing, it was created in 2009 which has its downsides. Phenomenonically, it'll always be the #1, but in terms of privacy (e.g.), it can't be compared with Monero (e.g. again).
In my opinion, without Bitcoin the whole industry would become chaotic.