when bitcoin is collapsing, everyone would lose trust in other altcoin fearing that it will also collapse, remember the ftx collapse? quite literally any coin that was previously backed and invested by ftx heavily hardly could rise nowadays because people already don't believe these coins anymore.
therefore if bitcoin collapsing, every coin will at least lose 90% of its value until eventually many abandoned them.
People basically invest in altcoins because they want to diversify their investments and not because they don't trust or like Bitcoin, and if they see that Bitcoin is collapsing, they will basically dump their altcoins first before they get an opportunity to sell their Bitcoins, so altcoins will collapse first.