I think of substrate as a sort of passive participant in faster plant growth - important but not by itself as much a stimulant to algae. Light and CO2 are different IMO.
I'll tell you why I clicked on this thread though: "downgrade". I have both CO2 injection ("hi-tech" in my book) and low tech tanks. I do not consider low tech a downgrade, it is just slower growth and cheaper. I belabor this point because I think the correct healthy outlook is required for either approach. Just click on the show-and-tell thread to see dozens of very fine tanks that are downgrades to no other approach. It has its own challenges so make sure you will enjoy it!