If my wallet doesn't support RBF, there will be no option for me in software to increase the transaction fee. What's the point there?
The point there is to use a better wallet. 
Network rules are not bound or limited by third party developers failing to implement basic functions in their software. Being able to replace an unconfirmed transaction is an increasingly basic function of bitcoin. If your wallet does not allow you to do that, then find a better wallet which does. Let's imagine we already have full RBF:
I download wallet X, it doesn't support RBF, so despite the fact that my transaction is marked as RBF enabled, I'm unable to use that option because of my bad wallet.
I download electrum, enable RBF and sign a RBF enabled transaction, I could use RBF yesterday and I can use it today, doesn't matter whether it's an opt-in option or not. If I want to use it, I can use it. It can probably benefit people who use Electrum but forgot to enable RBF but how common is that?