Battery technology is getting better every day, but the cost is still a big concern for many, atm you'll probably spend as much or more on batteries as you would on the machine itself, if you were to use them all day long.
But there's some things to way up. Cost of battery compared to service costs of a petrol machine, cost of fuel oil etc. There's not a lot of maintenance to do on a cordless.
Batteries need charging, and would require a few batteries to get you through a whole days mowing, charging them on the go isn't really there yet. You could use an inverter in your vehicle, but that would require you to leave the engine running, which isn't practical.
A power bank if there is one would have its own batteries inside it, they would cost a fortune, maybe a grand or so.
Regarding the mains electric, I think you have the best solution atm.