After level 10, the availability of new blueprints is a major limiting factor for upgrading great buildings. Motivate your neighbors' and friends' buildings, contribute to the same great buildings... there a plenty of ways to get more blueprints, but sometimes it does take a while.
However, HS being a low age GB, it should be fairly easy to get prints. If you're in EMA or later, HS will be one of the BPs auto-generated by the system when you get BPs from Incidents, GE, or DC. Before long, you'll have plenty.
In the bigger picture, I don't think the Hagia should be your first building to level over 10. what other GBs do you have?
It really depends on your needs. In my LMA city, I currently have HS at lvl 11 and will soon have it at lvl 12 because I need more happy. My city is enthusiastic now, but only because I have a Mighty Maypole down. However, I'd like to put the Maypole back in storage, and I don't like depending on happy buildings other than HS. Besides, every other level gives me another FP. HS gives the same number of FPs at every level as both CdM and Inno Tower and is the cheapest FP GB in the game by hundreds and thousands of FPs than either of the other two.
If, like me, the OP isn't ready to build Traz and wants more happy without culture or decos, HS is a fine choice.
