Sounds like a good idea since its for "Old" tomes would help vets gear multi-sub classes faster (or the grind on u'r 1st Relic... or 4th, whichever) & as the bonus is givin to everyone it helps the "possible" barly geared newbee get better gear faster. Add that "hard" cap to the bonus (like OP idea or a slight variation) & it sounds great.

Sad part is in most cases ppl leaving dungeons at start an't probably gona stop, to some no "old" tome bonus is worth it, but it might lower the percentage by a bit... maybe, doubt it but still maybe