Ads if you don't want to pay, no ads if you want to pay. If there is any problem with that, I fail to see it. The only thing I can really think of is that it is

Ads if you don't want to pay, no ads if you want to pay. If there is any problem with that, I fail to see it. The only thing I can really think of is that it is not economically worthwhile - the expectation is that so few people will be willing to pay or leave because of ads, that the added complexity is not worth it. You could then try to throw in some random stuff - larger file size limits, more GIFs, different skins, or whatever might work and is not too expensive - in order to motivate more people to pay, but this might again be an opportunity for a competitor who could offer your bonus stuff also to the masses that are happy with ads.