Timing-wise, it was too late for what they ended up doing, but anything all those people worked on is bound to have some interesting ideas.
It's been weird watching the rise of iOS loosen the reliance on, or even awareness of, files, when 9P was all about files. Files for everything!
I had the impression that it was held back by its proprietary license.
That was my impression, too. And then it got relicensed to a FOSS license... that was GPL-incompatible. And then it got relicensed again to GPLv2, and then it got relicensed yet again to its current MIT license.
In any case, each of these license changes was too little too late to really improve Plan 9's practicality from a "can I legally use this?" standpoint.
But basically, the hardware support was pretty bad. It took me a long time to find a SCSI controller which was supported, when ATA disks were already standard for years. Same with network- oder graphic cards.
Nowadays, if esoteric OS's would just support standard Vmware hardware, they'd be much more successful (looking at you fuchsia!)