Micah Sweet
Level Up & OSR Enthusiast
Level Up has its own character builder: levelupgateway.com. no need for anything from WotC.Class structure and mechanics between the two version is pretty minimal. That support is limited to people that have already purchased the old version. The old core books are invisible to new subscribers as far as I know.
Now, if D&DBeyond starts to support Tales of the Valiant or Level up in the marketplace of character builder you may then have a point.
Even then it is still 5e the core structure is similar in all those cases.
Now if they change to game to the degree that 4e changed from 3.x. that would require some extensive work on the D&DBeyond codebase to support it. It would also create a strong incentive for people that do not want to play the new game to switch out to one of the 5e clones. Then they are creating a real incentive to not continue in the D&DBeyond ecosystem and marketplace and move to platform that supports the ruleset the players prefer.
Then they could try to retain the old customers by supporting the old game with new material, or invite the clone games onto D&DBeyond.