Seems like the topic of upgrading groupsets is an age-old question!

I live in Boston and predominantly bike around the New England area so the terrain is fairly gentle in terms of topography (unless I'm tackling the blue hills observatory service road). I have found that my current 3x9 groupset is quite comfortable for me. I have ridden a bike with a 2x11 groupset and it was quite fun. Though that may have more to do with the fact that it was a modern carbon frame bike and the trip was in Maui. I was surprised by the range at which I could switch gears before worrying about cross-chaining, and I found the combination suitable for tackling steep climbs.

So it sounds like if I were to upgrade to modern Shimano 105 it will be a significant investment, at which point, it may be worth just looking at getting another bike if I want to spend that kind of money. I guess the odds of me finding an older generation Shimano 105 9-speed cassette and 3-speed crankset (new) are quite low. The teeth are unfortunately quite worn as I tried putting on a new chain and it kept skipping so I had to reinstall the old chain which is a hazard, and hence the desire to upgrade parts of the groupset. It seems my other option would then to be to "downgrade" to shimano's "Sora" or "Tiagra" modern components depending if I wanted to stick with the 3x9 setup or as some of you suggested switch to a 3x10 setup.

I would assume mixing and matching groupset components isn't ideal, but what have been your experiences?