Once you get the perks you want you totally can hold onto it. Every ten levels all of your perk and major attack stat bonuses on gear get more powerful, (but there's no stat bonus to upgrading before the 10 level threshold at all) even when you upgrade the same old gear you had, but it happens at the "first" level of each ten, so not at level 20, instead it's 21, 31, 41, etc.
Basically I've had the "same" gear for like 30 or 40 levels, now, just every time I hit the new stat range I go and upgrade all of it. It costs a shitload of gold, but aside from ship upgrades I don't have another major gold sink, so it's worth it to me, my perk loadout is extremely optimized for assassin damage, which is important for me since I'm basically playing it as an open world stealth game where I only fight if I get caught, I can one shot anyone I want, no exaggeration (using critical assassination when necessary).
The final ship upgrades are very expensive, yeah, just seems like they give you something to grind for if you get that far