The correct term is, it becomes slower because of the problems of Bitcoin like the tx speed and the fees of it but still the number of merchants accepting is still increasing. I can agree with what you said that merchants are accepting other cryptocurrencies because of the faster speed and lower fees.
Bitcoin will be universally adopted if the above problems of Bitcoin will be solved and right now, it is slowly being solved. Lightning Network has been used by some people already but some aren't accepting bc1- prefix addresses and this makes the adoption of LN slower. If more people will use LN, it can solve the one problem of Bitcoin which is the tx speed.