If it would be infinite, then how would you get the benefit if there is no end to it? Is not same mechanic as a city generating growth / mana / research etc.
Intended mechanic, I don't see a problem with it
Tho, if you're researching the repeatable tech in 1 turn (meaning you're so pumped out on research & everything else) how comes you did not win the game yet? Did you raze all your enemy cities leaving them only with the most undeveloped one to make sure they can't attempt spell of making, and nuked all their armies to make sure they won't even think about Epic Quest? (as conquest is out of question since you can control that pretty easy at that level of advancement, and you will just refuse to join an alliance to deny them this VC as well).
As far as I'm aware monsters / lairs don't respawn over time, if you built and researched everything, why continue? Sure you had all map scouted long before you even got into middle of your research trees.
But hey, each to their own