Well, that's the thing. By the time I'd got all the techs and reduced my Science funding to zero, I was running 70% Luxury ( +owning a monopoly of 6 Luxuries), 140 Units over my allowed amount and still turning a 500gp profit per turn. So there was, interestingly, no actual benefit in selling them for extra gold, it would make no material difference to my game, even though they had become useless.
Even though I sold my Library, Temple, Colosseum, University in my Capital it still hit the 20k before I could get the two 66% totals for a domination win. I hadn't even built any Great Wonders at my Capital since Sun-Tzu. It's now about 1965 and I'm at about 50% land and 60% pop, still slowly moving my troops from island to island, even with my army now consisting of purely Mobile Artillery and Armies of Modern Armour.
I guess the slowness can be mainly attributed to my desire to keep each turn's micro-management at a micro-level and just having one battle-zone rather than maximising my gp to produce enough troops to take two or three islands at the same time. The other aspect slowing me down is my desire to complete this process without losing a single troop.
Admittedly, I lost a few Medieval Infantry in the first war to gain control of my island, but since then I've lost no-one, making wars so much more feasible in a Republic. Well, I did lose one solitary Unit. Would you like to know what that loss was? Why, it was a Veteran Mechanised Infantry attacking a Veteran Persian Immortal that was Fortified in a City on Plains. Just before my Units were all pure Armies.
So, anyway, yeah, there's no reason to keep the Libraries and Universities, but there's also no good reason to sell them, if that makes sense.