Gliders should make themselves conspicuous on the universal standard, not expect others to have to detect their niche protocol.
I do agree with the fact that gliders should get ADSB Out, instead of everyone else having to conform with their kit but they will argue the toss that it increases expense, weight and complexity for them. The other snag is that ADSB doesn't give them any collision avoidance guidance, whereas Flarm does and I can see why that is important to them when there's 3 000 of the buggers whizzing around in a circle.
I suppose the most elegant solution is for their Flarm gear to pump out ADSB as well.
FLARM has been around since 2004 and is fitted to a very high percentage of gliders in the UK and the rest of Europe. Affordable, low-power ADS-B out in the form of CAP1391 has only been around since 2018, so it'll take a while for it to become widely adopted.
FLARM does offer a unique collision avoidance capability for gliders, so I think that ADS-B will supplement, rather than replace, FLARM for the time being. It would be very nice if FLARM were to transmit CAP1391 ADS-B out, but CAP1391 is currently limited to the UK, so FLARM probably don't see that as a high priority.
I have both FLARM and a SkyEcho 2 fitted to my glider.