MIMs are much better these days than they were back in the '90s (when I bought my MIM Jazz). Every now and then, I feel like I want to get a P-bass and when I A/B MIM Ps vs AS Ps, they sound pretty much the same to me. The pups in the USA P might be a little hotter, but they both have that distinctive P-bass tone. I wouldn't feel like I was missing anything at all with the stock MIM pup (and I'm a compulsive modder).
It's easy to think the grass is greener with a 'better' or 'more expensive' set of pickups. Or to assume that just because you have a less expensive guitar, the pickups must be junk. In the case of my MIM Jazz, the pickups didn't sound 'bad', they just didn't sound very exciting to me. On the other hand, a bassist friend of mine loved the sound of that bass and was always asking to borrow it for gigs (I ran sound for that band, so it wasn't a big deal). Good tone is in the ear of the listener.
I eventually ordered a set of Bartolinis (and waited 6 months for them), thinking they MUST sound better than my crap Mexican pickups. After all, don't a lot of boutique builders use Barts? Don't a lot of people rave about them as an upgrade? They cost a lot of money, so they've gotta be better, right?
I was less than impressed and so was everyone in my band. Popular consesnsus was that my SX Jag with Fender '75RI Jazz pups sounded worlds better, so the Bart-loaded MIM Jazz got relegated to back-up status.
Conversely, on my most recent build, I ended up going with a 'cheep' GFS pickup over a Seymour Duncan that cost twice as much. I wouldn't say the SD sounded bad, I just liked the sound of the GFS better.
My Epiphone Classic IV Thunderbird comes stock with USA Gibson pickups. I never cared for Thunderbirds before because I didn't like the tone or the clubby neck. For some reason, the Classic IV sounds 100x better than the Gibson bird (to my ears) even though they have exactly the same pickups and construction is identical. I like the feel of the neck better too, so it's all win. But a lot of T-Bird owners are going nuts for these vintage-replica pups made in Hong Kong, swapping out their Gibson pups for them. While I'm tempted to try them, I already LOVE the sound of that bass with the stock pickups, so why should I change them?
To sum up; yes, pickups can make a BIG difference, but it's not always a positive one - more expensive doesn't automatically mean 'better'.
Emotional bias can color perception. If one thinks their bass or pickups sound like crap because they're not expensive enough, then not much is going to change their mind until they start listening with their ears instead of their wallet.
If it ain't broke, don't fix it. You must have thought your MIM P-bass sounded good when you bought it. What changed your mind?