The cost is time.
True you can swap abilities on the fly, but builds are defined by more than just chosen abilities. Gear, legendary gems, actives/passive skills, cube powers, and if you want to stay competitive, augments.
Not everyone has ancient or better of all class sets lying around in their stash. That’s time spent farming new set pieces.
If you don’t have the right legendary gems leveled up that’s time spent leveling them up.
If you are changing gear, chances are you don’t have them all augmented, and that’s a lot of missing main stat. That’s time spent leveling up more legendary gems to augment all your gear.
If you don’t have a stack of Ramildinis Gift, that’s time spent farming it for the extra weapon affix.
If your old build had all this, and you switch builds without doing all this work, you will be severely under powered.
Technically you can switch builds with no cost, true. But I’d argue that’s it’s not really free, if you are wanting to solo GR push, there is a cost at getting the new build up to the speed of your old build, and that cost is time.
In fact I ran into this exact problem. I have Tal/Deathwish build on my wizard. I tried switching to Vyr/Archon, but I could not do anywhere close to the same GR, because of all the missing augments on my new gear.