First of all, they need 15 cars before they will start with the "EVRX". So if the manufacturers spend too long time making the cars, or too few are interested, it will not start in 2020 from what I can understand.
Secondly, WRX
need to do something, and quite quick. The costs are getting out of control and they either need to get the costs seriously down, or get an influx of new manufacturers. Going electric might be the way to get more manufacturers in. It's virtually impossible to compete as a privateer in WRX now, which is why you see many guys stepping down to ERX and national/regional championships (This is great for RallyX Nordic though!). The whole situation is quite similar to the last half of the 90's and beginning of 00's.
The "Rallycross Specials" that were introduced after Group B in 1993 worked quite well in the start. There was quite a few different cars on the grid, and the grids were quite healthy. However, as the years went by, the costs got higher, the grids smaller and interest gradually fell. At the same time however, the Norwegian-Swedish Championship was formed. You got all the top drivers from the European Championship (with the exception of Jean-Luc Pailler) driving there, and even got drivers who retired from ERX to just do the Norwegian-Swedish Championship. That one got shut down in the end, apparently in a ban on "multi-national championships" as it was getting very popular with all rounds being shown in TV in both countries as well.
However, the ban didn't force the drivers back to the European Championship, but instead they decided to retire. When you look back at it, the 2002 season had 6 full time entries, and quite a few races with a lack of wildcard entries as well, some races only had 12 entries, other that had more, seemingly allowed whatever car to drive in "supercars" to boost the grid.
Anyway, we are creeping towards the same situation now. The budgets are rocketing, you need manufacturer support to do something, and the bubble is about to burst.
In regards to EV's in RX, that might be the racing series EV's fits best in to. Lots of torque, instant power, short races etc. The cars of today are not much faster (if any faster) than the ones of early 90's.
It's slightly hard (if not impossible) to outright compare them, as the circuit records back in the day was not a single lap, but rather the fastest ever time over 4 laps - a qualifying heat. And the comparison is impossible to make after the Joker-lap was introduced. However, while Martin Schanche set the first non Group B record at Valkenswaard in 1994. Afaik, his record at Lyngås in Norway, never was beaten. So when the track closed down some years ago, the offical record was still from a Gr.B car.
Who knows, an EV might actually be faster than the old Gr.B cars
Ideally I would like manufacturer support to be banned (if that would be a thing to enforce), and rather market RX as the last, true racing series for the passionate privateer. RX thrived on the many ingenious solutions thought up by garageristas around Europe. It would have to be slightly overregulated in terms of budgets and so on, but I think that would an awesome thing. However, I do see how unrealistic that is, but a man can dream!
Anyway, EV's are the way the world is going, and the kids that are born now, might be more used to silent cars, and lower noise levels than we are, and thus they might not care that much (if all) about silent racing.
Personally I would like EV's to take a hike and have the combustion engine do a massive comeback in popularity. I have a couple of old cars I've invested in, that will be rather worthless in a few years the way this is all heading. So much for that investment!
TL;DR: It can work with WRX, they need to do something radical to save the championship!