There have been more than 19400 cryptocurrencies but strangely no one has come to an idea to restart bitcoin so to allow new users to buy and mine it from the start.
That is not bitcoin v2 or anything you called it, it would just be an altcoin.
What stops a computer programmer to copy the entire bitcoin code and change only the limit from 21million to 20 million.
If bitcoin supply should be changed, it has to be proposed and majority of bitcoin community (I mean the miners) would have to support this before it can happen. Bitcoin should remain 21 million. Bitcoin has limited supply and it is deflationary, no need of altering the supply like centralized coins like ethereum and binance coin.
Yes there have been forks like bitcoin cash or bitcoin sv but they dont start from 0
You mentioned already, that they are forks. Bitcoin cash is a hardfork of bitcoin while bitcoin sv is a hardfork of bitcoin cash. They are also all altcoins. You can start your own altcoin project that will have total supply of 21 million but it would also be an altcoin, not bitcoin.
Satoshi himself stated that bitcoin is only one of the first, there might be superior versions of a cryptocurrency.
I do not know about that and that is probably incorrect, but bitcoin remain the superior as of now.