On the large end of things, it seems the norm is to use a diesel electric scheme for trains (or diesel hydraulic for mobile launchpads). Presumably for the reasons you cite, i.e. being able to make torque to get the thing moving and not break stuff (and still the electric/hydraulic motors need gearing). But they also need large engines to overcome the additional losses as well, or live with lower top speed, they don't "enable" a smaller engine per-se.
Moving something vs meeting equivalent acceleration and top speed parameters are not the same thing. If you have a 20hp motor and it can get your scooter up to 75mph with a gearbox, it will be lucky to do 60mph with a hydrostatic transmission, so you need to add a LARGER engine to get back to 75mph. An electric transmission (generator drives motor) like on a train would likely have more slippage and losses, and each half (generator,motor) has different peak efficiency parameters (load and rpm) most of the time.
The gearbox allows you to stay fairly close to peak engine bsfc without constant conversion losses, for vehicles suitably sized for a gear box. If you look at a somewhat typical bsfc map (i.e this
saturn) you can see that you can stay within the 250g/kwh peak island from below 1500 rpm to above 3500 rpm. And the transmission does NOT drop to 1500 when you upshift from 3500.
As well with no batteries to charge, heating resistors just to run your diesel near peak bsfc is a losing proposition. I am having trouble thinking of a scenario where it would be better to increase the power and fuel demands of the engine to make waste heat and somehow expect that to use less fuel, given typical bsfc maps.
And charging/discharging batteries is not an efficiency free lunch either.