So, you dont have to slow down your expansion for decades if you are willing to sacrifice a percentage of your income to combat corruption. Keep in mind, there are other ways of expanding as well. You can expand via vassals/personal unions to hit diplo instead of admin. You can now take territories instead of making them full blown states to cut the cost in half of taking territory if you want to level your admin and still expand a bit. If your willing to fall behind in military and diplo tech by a level or two that also increases your expansion capability.
Once again, it comes to trade-offs, you still have options, you don't have to be full tech in everything, if your willing to fall behind you can expand more which might be worth it.
Edit: Think about it, so your two levels behind your neighbors in military tech, that gives you potentially two more levels in admin tech you can fall behind. Now you, in addition, are willing to take on a level or two of corruption, you can still be 6 levels behind your neighbors in admin tech, that isnt going to delay your expansion by "Decades" it just forces you to make tradeoffs to do the same level of expansion.