Originally Posted by alcjphil

There are 2 things that come to mind for me. First, good quality derailleurs are not unreliable. I used to commute to work, a 60 km round trip that I used to do 5 days a week, up to 9000 km per year in Montreal where winters can be quite long. Maintaining a bike with a derailleur consists mostly of keeping the chain relatively clean, adjustments are fairly rare, usually only needed when changing worn out cables, cassettes, and chains. I used to have to change chains about 3 times per year, cassettes once per year and cables about once every 2 years(downtube shifters). This notion that shifting goes out of adjustment after a couple of months of use sounds odd to me and makes me wonder about the way the OP is operating their bike.

Next, another point is about the bike itself. Nowadays, 7 speed rear shifting is relegated to lower quality bikes. I do own a bike that has 7 speed shifting, but it was a high quality bike when it was built back in the early 1980's. It's shifting is reliable and predictable, but it was probably a $1000+ bike 40 years ago. The OP's bike looks like the sort of thing you would buy in a discount department store. I bought my first derailleur equipped bike in 1972 and I rode it heavily or a couple of years trouble free. However, I bought a much better bike the following year and rode it for the next 27 years until damaging the frame in a road race crash. The derailleur components were transferred to another bike frame where they served me for several more years until I upgraded to more modern shifters. I still own most of those components today and they are still seviceable after well over 30 years of use.