The game actually tried directly selling champions (Hordin and Mother Superior iirc) before but everyone agreed that it wasn't a good idea. 

The rares needed for the fusion are all attainable through events and tournaments anyway. Mid game f2p players can do fusions just fine. If you're early game though, there's a lot more productive things you can do for account progression. 

PS believe me if you were around during the first few fusions where some events were paywalled, or where most of the rares needed were new champions, you'd agree when I say the fusion now is absolutely fine. 

For early to mid-game players, it is almost impossible to complete the events and competitions to get all the Characters to pull off the Kantra fusion. If you haven't completed BRUTAL campain and can't farm the best levels, you can't expect to reap enough XP, silver and artifacts to reach the later rewards for those events and competitions.

For many early to mid-game players, getting a good Legendary is probably a stepping stone to reach and beat higher content. Like myself, many of those players invested a lot of time, energy and probably money (for shards, gems and energy) to push throught those events and competitions. Some bad luck or bad timing and you can miss out a key champion. Loosing everything you invested in that fusion event.

If there could be a way to get those champions outside the events, it would be really great. I know you can get those champions from Sacred and Void shards, but the result is pure random luck. I'm sure a lot of players wouldn't mind paying a few bucks to buy the missing champions, to level them up and then complete the fusion.

There are 16 rare champions to get. If Plarium would charge like 3,99$ for each one of those champions, if would cost 64$ to get them all (no levels or ranks yet). So buying from scratch would be quite expansive. But if you just need 2 or 3 champions, it could be worth it.

I'm also sure that a lot of free-to-play players would jump on this occasion to have the satisfaction of completing that fusion event.

From my point of view, Plarium and players would all be winning from such a feature.

Thanks.