Unless F1 and FE merge, F1 cannot become fully electric until the end of the exclusivity period. It's a gamble by F1 that the world (and more specifically, the teams and manufacturers involved) won't be ready for a full-electric pinnacle series by 2039 (i.e. speed and name recognition parity).
All F1 teams have signed the Concorde Agreement until 2025 - which means they're "committed" in one sense or another. I don't think FE will be in a position to attract Mercedes and Renault fully by 2025 (the two mass-market manufacturers who'll be most heavily influenced by the move to fully electric, with toes already in the waters in FE).
The late 2020s will be a lot of political posturing, and should FE look to be attracting F1 backmarkers with promises of cheaper overheads, more future-relevant tech and decent audiences in addition to manufacturers, 2030-2039 will be interesting as heck.
