Agreed, but you will have to essentially cut and remove the entire back metal where the slots are to accomodate a standard ATX 8-slot motherboard, as well as drill and finish new screw holes. Destroying a classic case just so you can put in a modern motherboard seems tragic, but then again, it is yours and you can do with it whatever you like.
If I were to pursue this project, I would try to trade my 5150 for a 5160 with someone else. That way there should be little to no modification required of the case, although you will still have to make at least an additional hole for the PS/2 mouse connector and cut out a space for the other back connectors on the board. But the main slots/holes should line up.
Don't worry. I will never touch my 5150 (or any other of my classics)!
I may end up saving some cash to buy a 5170 case with PSU or a fully working one.
Thank you.
Jose