I spent about a thousand hours or so doing tours in 172's, plus some rough-field, back country work. Out of all the airplanes we operated, which were 172's, 206's, 207's, 210's, and the Super Cub, the 172 had the highest profit margin, because it was so inexpensive to operate. The 172 is very economical.
Likewise, the 152 is very economical. It's not an expensive aircraft to operate. Especially given the low acquisition costs compared to more "modern" aircraft, the 152 or 172 can be operated for a very long time before the cost differential catches up. Personally, I'd rather spend that time flying and for the person putting their airplane to work, I'd much rather have it paid off and making money, than hanging over my head as a major expense.