And people make a big deal about a single alternator, but it is not
true that it is single point of fail. It is also backed up by the
battery.

Right, the single point failure is not the single alternator - it is
the single battery contactor. If that fails (or the battery cooks
itself) the alternator ALSO goes off line - because it needs battery
power to excite the field and won't self-excite. Most light twin
electrical systems feature two alternators - which isn't any better,
because the battery/master contactor still exist as point failures.
Most generator-driven light twins have paralelling relatys, which, in
the event of some kinds of failures, will cook the system. A truly
redundant (no single point failures) electrical system is a very rare
thing in light GA. That's why all-electric airplanes scare me unless
they have independent buses and multiple batteries.

It seems to me that an electric attitude
that flags itself for problems is the best you can do here.

No such animal in GA, I'm afraid. You can buy one that flags loss of
power (be it vacuum, pressure, or electric) but that's not the most
common mode of failure for the AI - generally it is the gyro mechanism
(brushes, bearings, etc) that fails.

Ultimately, the only solution is dual independent power sources
(neither alternators nor generators with a paralelling relay qualify)
and dual attitude gyros. For example, one vacuum and one electric
attitude gyro is a great choice. Few GA airplanes have that.

Michael