I think the scenario that you really want to discuss is what if all the altcoins disappeared instead of some sort of merge, however if that happened then I do not think the market cap will go as high as you think it will go, the market cap of most altcoins is many times lower than what you believe, however the real benefit in that scenario will come from the fact that people will not be scammed by shitcoins anymore, and as such they will invest more money in bitcoin as they will not experiment the losses and the trauma of losing their money to scammers anymore and they will have a higher trust in this market.