I kind a agree with Ramabaten on this. Steam only would be a preferable option, but after 5 years I do believe it is too late as our core is nonsteam.
About fully moving to CS:GO.
First of all, CS:GO requires at least a good pc to be played smoothly. A lot of people have problems with CS.1.6 as it is.
Secondly, CS:GO has online matchmaking which also reduces the need for servers. They can be made of course, but there is a lot of people who will just play that.
And thirdly, Ambrosia from the start has always been cs 1.6 orientated. It was started due to cs 1.6. So pay your respects to the game. This game has been played for almost 15 years from 1.1. It didn't die as some games do. A lot of people might stop playing it or get bored of it after a while, but they tend to start playing it again.
Also, because of few, you wan't to move fully to GO? Consider the people who cannot afford it, doesn't have the requirements to play it and generally are non steamers.
Additions to future CS:GO servers yes, but not fully moving to it. Would be a reckless move. Besides, Ambrosia has what, 10-20% of CS:GO players who own CS:GO?