I don't think it's carbon shafts necessarily but they're helping move the process along for sure.
The first step was LD shafts. Before that everything was basic maple and ferrule material, taper, wood selection, etc all factored in greatly to playability. Then LD came along with mass manufacturing techniques, greater consistency (though not perfect consistency), and a performance advantage in their lower deflection.
As people started to adopt those fairly widely there was some lessening of the importance places on custom cues I feel like. Many people started taking their custom cues and adding these LD shafts to them. I do believe the butt plays a factor in balance and feel, but in actual performance when striking the cue ball I don't think so.
The move to carbon shafts added greater durability and consistency but really did add a lot to the performance of LD shafts. But, they added a visual element where they just look so drastically different from a standard cue. As more and more players started adopting the shafts I think the visual change became a big factor. Carbon was seen as modern, performance oriented, consistent, and durable.
That shift also seems to have happened alongside the rise in popularity of screw on rear extensions and while some custom builders are offering those now, and I think all will have to in the future to stay relevant, the production world was on top of that move sooner and with better solutions in a lot of cases.
I think that has all led to a shift towards production cues for a lot of players, or to a smaller group of custom cues that offer these modern features that are then paired up with a carbon shaft. In most of those cases the custom butt is for the visual aspect, respect for the craft of making a handmade cue, and collectability/exclusivity.
I get all sides. I've been playing with a full Predator setup lately and really liking it. It plays great and offers some clear performance advantages. I've spent some time comparing my new rig (K Series Classic 1 cue w/ Revo, BK Rush, and Air Rush) with my previous rig (Runde custom cue, OB break cue, Jacoby jumper) and in all areas the Predator has a performance advantage in my experience. I love having easy screw on extensions (have 8" and 3") and their adjustable weight setup is really quite nice as well. The durability of the carbon shafts and their easy replaceability should something happen means I worry less about them getting knocked around a little while at league. If something happened to my favorite Runde shaft for example I'd be devastated as I'm not sure I could replace it.
But, at the same time, I'm holding onto both my custom Runde w/3 shafts and my South West satin w/2 shafts. I actually had the South West out for a couple days in the last week and marveled and the feel and hit of that cue. It really is a special feel and it moves the cue ball so well. It has a fair bit of deflection and I can adjust, after a time, pretty well for it but I still wasn't making shots with spin as consistently as with my Revo. I also didn't have the ability to quickly and easily add an extension for a bit of added reach.
So for me, I'm probably going to be sticking with my modern production setup for general playing because I really do think I play better with it because of the performance advantages. But I'm going to hold onto a couple customs too because I like them, appreciate the craftsmanship, and because they do tend to feel better IMO.
In the future I think the custom makers that thrive will embrace the modern while still offering their custom touches. Cues built with the knowledge that a user is likely going to add a carbon shaft and joints designed so that they'll fit properly. Perhaps even embracing some of the existing standards for screw in extensions. I love playing with my Predator setup currently but had I been able to find a nice custom that could take a carbon shaft and good screw on extensions easily I likely would have gone that route instead....I like nice handcrafted items and I'm willing to pay extra for them even if they aren't offering me a performance advantage.