Truthfully I don't really see what this adds.
Civilization is a game about making interesting choices, choices are interesting because they have trade-offs.
With this proposal, if there is a city I want deleted, I now have two choices.
1) is raze it, which is costly, gives me a diplo penalty, a happiness penalty, and causes partisans to spawn
2) plink at it with ranged/artillery until it dies, I get to farm risk-free (risk free because if I'm conquering a city I have map control around it, so my ranged units may take some bombardment damage but are otherwise at no risk of death) XP, I receive no diplo or happiness penalty, and there are no partisans. The only disadvantage I see is this takes longer than razing but I can just keep my siege units chugging away at it as my army pushes to the next city as they have to wait for map control around the next city anyway.
Right now this option is better in every way than razing, that's not an interesting choice.
Maybe if you add happiness/diplo penalties/etc then they're both more equal, but at that point you've just added "razing 2" but now with a bunch of additional coding requirements for tactical AI, penalties, etc to re-create something the game already does.
This is just scope creep for scope creep's sake, this doesn't meaningfully expand the option-space of the game.