I believe the first gyros, for turn indicators, were venturi operated. It was soon found, the hard way, that venturis were prone to icing (a dim memory says that an iced-up venturi was involved in Knut Rockne's (famous football coach) death in a Fokker). But (I think) the same instruments could be used with engine source vacuum. Electrical systems came along for navigation light and cockpit lighting, and then artificial horizons which if operated electrically gave redundancy, anyways the vacuum was more reliable and the turn indicator gyro didn't topple.
Some may have read of a certain Convair 240 (?) which took off from Boston circa 1950 +- a year or two in IFR conditions at night, with all gyros operated electrically, and suffered a total electrical failure. Luckily the captain was able to manage in a rough way with the magnetic compass and staggered along to the New York area where visibility improved and they found a lighted airport. I think this reinforced the idea of vacuum plus electrics.
Now of course electrical systems in aircraft are almost perfectly reliable.