Mana sustain idea

Why not increase mana sustain the longer a character goes without using mana? So it would work like this:

0: Uses ability, mp5 reset to base 15: after 15 seconds of not using mana, total mp5 rate increased by 75%

Then, every 5 seconds afterwards, total mana regeneration increases by 25% until reaching some cap. So if Joe Bob has 7 base mp5 and 100% bonus mp5 for a total of 14, then this system would give an additional 10.5 mp5 after 15 seconds and 3.5 additional every 5 seconds following.

With this system, mana is still limited during fights and one-after-another engagements, but it allows for mages to revive more during laning if they just use a spell or two per wave. It would keep the differentiation between mana and other resources for drawn out fights/sieges while not being so punishing for the rest of the game.