I do think altcoins disappearing will be a net positive for bitcoin in the long term. I think that it will make people realize that 99% of projects are just purely fluff, and what mostly matters is bitcoin. People are just creating blockchains for literally anything that it's beyond stupid. Think about all those scams that left a bad taste on people's mouths.
Altcoins exist because they fulfil a need that bitcoin fails to fulfil. For example, I move money from exchanges using alts simply because the withdrawal fees on bitcoin are too large. There are never any mempool backlogs on alts either, so you can move coins like Litecoin or Doge swiftly and then exchange for fiat on exchanges like Kraken.