That's the theory.
The practice is ... different.
It doesn't matter matter if there are only 50% as many parts to service if servicing those parts costs three times as much.
At the end of the day, this is all revealed via depreciation.
Cars experiencing a lot of depreciation, by definition, are viewed by the market as more expensive to service over their usable life, as depreciation is just the forward payment of future maintenance costs:
Say a car will last for two periods. In the first period, the repair costs are zero, and in the second they are 15K. Now the car costs 35K new. Total ownership costs for both periods are thus 35+15 = 50K for 2 periods. Split evenly, that's 25K per period. But the buyer in period 2 will pay 15K in repair costs and 10K for the car. The buyer in period 1 will pay 25K in depreciation. Both parties pay 25K. If they didn't, and one party was paying more than the other, the put-upon party would prefer to buy new and resell instead, or buy used, etc, so in equilibrium, both parties pay for half the purchase price plus half the lifetime repair costs, except the new car buyer "pays" for their half of repair costs via depreciation whereas the used car buyer pays out of pocket.
Now you have a situation where Tesla Model 3s are depreciating at rates comparable to the dreaded BMWs -- notoriously expensive to maintain cars. E.g. they are losing 44% of their value after 5 years of ownership (source: https://caredge.com/tesla/model-3/depreciation)
So it doesn't particularly matter that there are fewer moving parts, etc, as for some reason the market believes these cars are going to be about as expensive to maintain as BMWs, which have lots of moving parts (and lots of plastic parts).
Now if we know that, we can come to a few conclusions:
* the market is just wrong and out-of-warranty Teslas will be cheap to maintain, in which case there is an arbitrage opportunity to sell Tesla warranties and make lots of arbitrage profits. Feel like getting into that business? Me, neither.
* The theory is wrong and the fact that there is a simpler drivetrain doesn't have much to do with overall maintenance costs due to all the other stuff modern cars do that don't have to do with moving down the road. Uhh, maybe.
* The theory is right and the electric car makers are just screwing the pooch by managing to sell expensive to maintain cars even though there are fewer moving parts. I tend to favor this theory.
But in either case, arguing that because there are moving parts, the cars must be cheaper to maintain is a fallacy. It's a confusion of theory and practice in one of the many situations where out of warranty car owners are screaming in pain at their repair costs, and countering "muh moving parts" is missing the point.