If you live somewhere long enough and under a negligent enough landlord, you can just do a lot of those upgrades anyway and either take them with you when you leave or just chalk them up to practice for when you own a place.

I've lived in my current apartment for 9 years and I've never met the guy who owns it now (it was sold). I'm also not getting my deposit back, so that doesn't matter.

It's the big stuff that's annoying. Can't install A/C or an exhaust fan in the bathroom, for example, simply because I can't afford it. I'd totally feel comfortable upgrading the stove/fridge and tossing theirs or putting it in the basement. They're not going to find out until I move out anyway.