The only problem would be the lack of bosses, so it would definitely be very different and probably far less rewarding. There's only 24 bosses in Act 6 vs the 64 each in Act 7 and 8. It'd be fun, sure, but probably wouldn't be that enticing.
Sounds like a fun idea, but I wouldn't expect the rewards to be too huge. Maybe 6 star R5 gems or something that would benefit players at Thronebreaker or Paragon levels.
Honestly they could add that as an extra challenge for people getting into the game, one for Act 1 + 2, one for Act 3 + 4, and if they need to expand the crucible one option could be to combine Acts 5 and 6 too, that would give a total of 48 bosses, which is shorter than Act 7 and 8 but still makes for a solid challenge if restricted to 5 and 6 star Champs
bad idea