As many people have pointed out: the Ottomans are not unbeatable, nor are they OP. They are strong yes, but really: as France, Austria or Sweden no moderately experienced player should have much difficulty with them, especially late-game. And yes, if you are looking to do a long-term game, you should be looking to slow down their advance even in early and mid-game, no matter how far removed from you they are.
Obviously many players do not share your opinion. In fact, for many of us, myself included: playing smaller nations around the Ottomans is a nice challenge. Playing Athens, Moldova, Karaman and George have been some of my most entertaining and exhilarating runs. Alternatively, the Ottomans often pose a nice late game challenge in such runs as trying to conquer all of Africa as Kongo, moving into Asia as a colonial power or simply going for Mare Nostrum. A challenging Ottomans is the keystone to many an EU4 run.
We’ve all undoubtedly have had many frustrating experiences in this game fighting them, but it is key to turn this frustration into a desire to improve, learn and become better. Which is basically what it gets down to: become better to beat them.
But until then, an obvious solution seems to be: play as the Ottomans.