Yeah, there's nothing terrible about square taper spindles. Hell, Phil Wood doesn't even make a splined bottom bracket. If there was truly something magical about it, I would think Phil would make it.

There are a few problems with ST: cranks can be a challenge to torque just right. Too loose and you can round them out as they shift around, too tight and they can walk up the spindle or crack. They can be a pain to get cranks off. I've never done any work on a splined BB, so I can't say for sure, but as far as I know, they have no taper, so those issues go away.

I suspect that Octalink was a pure marketing ploy. What better way to lock you into the Shimano world than with a BB/crankset that only work with other Shimano gear? Ok, that's not entirely true. They'd be more than happy to license it to a 3rd party manufacturer for a price. All of this was predicated on some dodgy arguments that didn't really amount to enough of a difference to make it worth changing a decades old standard.

ISIS is a response to Octalink. It's still snake oil, but at least it's non-proprietary snake oil.

Meanwhile, I can get square taper stuff at nearly all levels of cost and quality and they play just fine with different components (obviously I can't get ST 1-piece cranksets).