There's a lot to unpack here, but i'll give it a shot.
Is there any real harm if I just let Crabgrass take over 100%?
It tends to form clumps that leave exposed soil between them; not a uniform look, and gives landing zones to all sorts of other weeds.
will one (CG, KBG, CC) win eventually?
That depends on a lot of factors that change by the square yard. Creeping Charlie will win in shady areas if I had to bet. If one of the others did conquer sunny parts, it could be a battle fought for decades before victory. They can also work out a sharing agreement. Then again, your soil could happen to have perfect conditions for one but not the other, basically negating the guidelines above.
If I just take this low-maintenance approach now, will my KBG
percentage fall over the years to come?
Unlike most cool season grasses, KBG is a spreader, so it's a good choice for low-maintenance long term; filling in divots and whatnot for you.
KBG is harder to get established though, taking longer to germinate (up to 2 weeks of daily watering) and it's slower to grow deep roots. Once established, it can survive drought well; though it will look "dead", it will come back within a few weeks of rain starting up again.
What should I do to keep this simple and maximize my KBG percentage?
If I was only allowed to do one thing a year, it would be to apply weed and feed (or a related product) to the area in mid spring once weeds are up and running for the season.
If I could do another thing once a year, it would be to overseed KBG in early fall, without fussing about watering or peat or any of that, just throw down seed.
If i could do a third thing once a year, it would be to fertilize in early spring. Milorganite's a good long-lasting choice, but compost or starter fertilizer would work too. Might want to use starter fertilizer specifically the first time since it usually has a lot of secondary minerals and nutrients you might be missing.
If it were me, and i didn't mind putting in a couple afternoons to get setup to have a neglect-resistant decent-looking lawn, here's what i would do soon-ish, in addition to the regular steps above.
- Roundup the patches of thick creeping charlie.
- Pull up the biggest crabgrass clods so you can't see any at a distance.
- Run a course of starter fertilizer.
- Throw down compost on the dead spots where the CC was.
- Seed with KBG, double recommended amounts (money buying effort)
- Cover bare patches of seed with peat moss
Then you "just" need to water lightly 2-3 times daily until seeds germinate. This part sucks, so I would buy a cheap watering scheduler from amazon and an extra sprinkler or two. Reduce watering if mushrooms appear. Stop watering when it's tall enough to mow. You can buy longer green time with mid-late summer watering if you want, but it won't "need" it.
Understand that these are not the ideal actions for a perfect lawn, and I'm 100% sure there are many more steps or refinements you could make. I will offer that I moved into a dirt patch which is now a nice lawm, and it's unexpectedly satisfying, and though it wasn't a huge goal for me or anything, I'm actually kind of proud when people notice it.
At the low end of the effort scale, there's a huge return on investment. It probably takes five or ten times as much work to get perfect as it does not-embarrassing. A few afternoon projects a year and it could look border-line good. Good luck.