The industry is disagreeing with you. Vortex generators are light, easy to install, and have a great bang for the buck. You see vortex generators on Super Cubs and Husky
The best performing STOL aircraft are equipped with slats. I'm going to stick with my stance.

The industry answer is that VG's are common and slats are not.
I guess it depends on your definition of "the industry". There are plenty of designs marketed currently (the various designs of various Heintz STOL aircraft, etc) that have slats and they tend to be the best performers. Yeah, you're not going to cruise very fast but I keep getting told on here that the "average" GA pilot (at least by EAA Forums standards) are not looking to go fast and besides most STOL aircraft are limited by the LSA speed restrictions anyhow. The one major exception is really the Helio. It's the one "heavier" aircraft that I'm aware of with freakish STOL performance.

Slats that are fixed force a full time performance penalty
We're talking about an ultralight. The whole design of 99.99999% of ultralights is one huge performance penalty.

and movable slats are more mechanically complex than you think.
I highly doubt that since I just designed them for my turboprop and curse the decision to go with retractable slats. They are probably the most mechanically difficult system on the entire design and make retractable landing gear and the controls look like child's play.