There is plenty of that already. An estate in a city (or another wealthy region) often pays more than the lordship of a rural/mountain/woodland region. I know that most of my characters have taken pay drops when leaving their estate to take up a lordship.
I'm not sure what you need excess wealth for as a knight. Sending it to your family?
Being a landed lord on the other hand opens a lot of new venues for you even if your income suffers. Then again, lords of poorer provinces always receive additional funding from the metropolis.
So why be a knight if you can become a lord?
Beyond that, I can not imagine a sensible way of encouraging knights to not become lords when the opportunity presents itself. I am not saying there is none but I can't think of any.
If you incentivise lords to hoard knights instead of incentivising knights to maintain knightly status, you are basically screwing knights over.
I would hate to see a situation where a Duke refuses to land his knight just because he wants to maximize his profit even if there are lordless regions available.
On the other hand, if knights have real incentives to choose knightly status over lordship (like being able to do some fun stuff lords aren't able to do), lords themselves won't even need additional encouragement.
