A bike is a complicated machine. If you want simplicity, you should walk. But for cycling pleasure you really do need a bike.
So you need to draw an arbitrary line in the sand that is your benchmark for simplicity, and say "This is complicated enough, it's the sweet spot, beyond that is
too complicated". Where do you draw it? Pneumatic tyres? Fixed wheel? Gears? Cotter pins? Indexed gears? GPS? and so on. It's just a matter of personal taste.
Probably most people set their benchmark for a "simple bike" at the level of the first bike that really got them in to cycling. They can then spend the rest of their lives bellyaching about new-fangled rubbish, with the spurious cover that they are not luddites blindly dismissing things that they have never tried or can't afford, old men yelling at clouds, but are in fact searchers for zen like simplicity.
For me the sweet spot for simplicity happens to be - purely by chance

- the same as my first serious bike: a 70s 10 speed steel bike with friction shifters. A fixed gear or single speed is too hair-shirt. Hub gear too limiting. Indexing is an unnecessary frippery. Cotter pins may be simple but a cotterless setup is much better. The gooseneck stem is infinitely adjustable, no fiddling with spacers. All bearings can be easily serviced. No one needs more than 5 speeds on a freewheel block. Navigation can be done by writing down a planned route on a bit of paper and carrying a map. Low gears are for wimps. Toe straps are sexy. No one really needs brakes that work particularly well. I rode one for thousands of miles and it never did me any harm. If it was good enough for Eddy Merckx ...
Of course, I don't ride a bike like that. I've got one and it's a bit crap. I ride a nice modern bike with 105 R7000 because it's much better. Even if it falls a bit short in the zen-like stakes.