> And with full legalization, you'd have billions of advertizing dollars invested into marketing and research departments by companies trying to convince people to get high from their brand.

Legal competition is also pushing the THC levels up. The THC levels are definitely not going in a downward trend in legalized states like Oregon, Washington or California. Flower you can easily find 30+% levels of THC and with things like hash and other derivative concentrates you can find over 90% purity.