Let's not be too hard on SE about BLU and in general.

[Reynhart]

They have made quite sure to say that this was a solo job and that you can only do the old duties if you use a premade group because it would otherwise be too strong.

They said hat breaking the game was part of what make BLU fun. In 4.5, you'll probably have fun breaking ARR content, yes.

And they have said that one of the reasons they chose to go that way, was the fear that blue mages would be kicked out of duties because they would not have certain abilities.

You realize that this has nothing to do with BLU's level cap. Kicking is only an issue in matchmaking, so as long as BLU is excluded from matchmaking, it can end at any level cap. It's also what made me realize that customization could be treated the same.

Why not release it at lvl 60 and raise it to 70 at the expansion release?

Because lvl50 is the cap of the base game. And I guess we probably have some players that didn't buy HW nor SB. Those players will be able to experience BLU starting from January. If that wasn't the case they wouldn't have such clearly stated that "BLU only requires ARR". But, once you raise the level cap, even by 1 level, you go into expansion territory. And at that point, since buying SH will most likely give you access to HW and SB, you could go all the way to 80 pretty fast even if maybe not right at 5.0.