By theory if all cryptocurrencies were merged together, I think the effect would be minuscule to the point that most will not even realize that such happened. Before the creation of all altcoins, Bitcoin was the first crypto that entered and introduced everything. Its fundamentals were somehow shared with other altcoins- so we can say that everything exists because of Bitcoin.

Now like what most have mentioned also, merging all cryptocurrencies into one would result to it becoming an investment opportunity for everyone where it would be at the same position when BTC was first introduced.