The US airlines all purchased some/all of the "optional" safety features that would have made it obvious to the pilots that the system was malfunctioning.

A system they weren't told about, weren't trained on. What they have is an "AOA disagree" indicator, among dozens of indicators. There's no reason for them to see it or go looking for it in the example cases of MCAS upset where they're reacting to an urgent situation. The indicator might be useful for a maintenance squawk report.

Ordinarily sensor disagree would give you some idea what could be affected because you've had training to tell you this. Not in the MCAS upset due to faulty AOA sensor case.

Until the reports are complete, I think it's getting out over one's skis to say would would have been obvious to U.S. pilots.

Southwest and American did, but not United:

Of the three US airlines that have Boeing 737 Max 8 and 9 jets in their fleet, only two — American and Southwest — paid for these upgrades, the Times confirmed. A United Airlines spokesperson told the Times that its jets don’t include these upgrades because its pilots use other data to fly their planes.)

https://www.vox.com/the-goods/2019/3/22/18277694/boeing-737-...

"use other data to fly their planes"

What does this mean? Pure PR spin? Isn't MCAS a mandatory system?

I think "other data" means "not those particular two optional lights that let you know when there are angle of attack issues". But yeah, I think it's spin.

There is no regulation that prevents them from making safety-related components optional?

Nearly every option you could add to an airplane is arguably a safety feature.

What a great way to stop development of safety features. Just imagine how the automobile market would look like with that kind of regulation.