Solved

Can't upgrade headquarters

Just got to level 9, but all the buildings in the headquarters still say I need to be level 9 to upgrade,,,,,

Still can't upgrade.

If everyone where to stay in leagues and they where full. After a while Rookie, Pro & Elite tiers would all have level 20 managers, it's hurting nobody lifting the tier caps, long as they have the money to continue upgrading why not allow them.
Only problem that could arise there is an empty slot and a level 1 manager joins, they wont have too much fun until they get a few seasons under their belt and they might not stick around.

Yes, I don't think limiting things like the HQ is such a good thing. If rookie tiers filling up with high lvl managers does indeed becomes a problem then limiting driver lvl and design points would be better, but best would be making this a choice for setting up leagues and the ability to properly list and sort/filter leagues accordingly.

Although since this game already has and, due to the number of players, needs leagues it might be nice to look how it's done in football and the possibility to set up different division trees for leagues, or perhaps even one official for everyone and optional player created ones for more races and fun. The 3 current tiers could be tied to things like lvl, reputation and be an optional limitation which strength of player can enter (where) in leagues. But that might be just me, or my inexperience here, liking something like that. :-)

Hmmm, I did not know this.
So the real problem is actually that this isn't made clear anywhere.

If I have to be honest, I wouldn't stick around in a league where everyone was 5-6 or more levels higher than me, unless of course the XP gain is far higher.