More importantly, this mistake only affects highly unusual, malformed
data. I think that if IDNA decides not to follow Unicode's
recommendation now or in the next couple of years, 10 or 20 years from
now we would look back in time and regret this decision.
I don't think so. "We" could still change the decision in 20 years, and
not a single registration would be affected. The sequences causing the
behaviour change are *really* unusual - I don't know if software can
visually render them in a meaningful way, and I guess a native speaker
would just consider them moji-bake. So it is unlikely that anybody would
try to use them as input to IDNA in the next 20 years in a reasonable
application.