I have a 40D, with Tamron 17-50mm, Canon EF-S 55-250mm and Canon EF 50mm compact macro lenses. I'm very happy with the performance and quality of this stuff and it does pretty well what I want it to do -- provided I play my part properly.
I used to shoot with 35mm film cameras for decades and I became very familiar with FF 35mm and the lenses associated with it. The "crop" digital concept is rather new by comparison, but I do understand it.
I don't take sports/action or wildlife pictures. I like taking landscape, street, architectural and portrait pictures.
The thing is, I have recently become increasingly interested in FF digital photography, for the high quality of the images. I could just about afford to get a 5D (classic) if I sold my 40D camera and Tamron 17-50 and Canon 55-250 lenses. All my gear is in great condition and the 40D has under 15k clicks, so I'd hope to get good prices. I'd keep the EF 50 CM lens for use with the 5D, hoping to add more lenses later.
I am thinking of doing this, and I'd be prepared to sacrifice many of the 40D's bells and whistles, but I'm not sure if it would be an entirely sane thing to do! I'd welcome comments from, others.