Newer models will of course have newer technology. Many manufacturers seem to be moving away from naturally aspirated motors to smaller, turbo charged engines. You will probably get better gas mileage, but I worry about longevity of the extra bits such as the turbos and of course the costs associated with them. That being said, I’m not a fan of the front end on the R172s. I have a R171 and looks better IMHO. I love my XF as well. The technology isn’t nearly as advanced as the R172, but the “smiles per mile” factor is quite high still and I see no need to go down the R172 road.

You are correct that 2020 will be the last year for the R172. They just do not sell like they did before. Of course, MB has many different convertible models as well. The SL may also be on the chopping block as sales figures are down. IMHO, the newer SL design is just not as cool as it used to be in the past as far as design.