There are definitely much cheaper ebikes and conversion is also another option (I had no bike to convert). For me, I was willing to pay more to get everything I wanted in the ebike. I'll be commuting a decent distance per day and need something that can handle it.
Note that as an ebike I don't need to pay for registration, etc. Technically as a motorbike rider you're going to be caught in the same traffic as cars, although I know most motorbike riders are willing to either break the law or ride a bit unsafely to get around the traffic. On a bike, I get to use the bike paths, etc and can legally beat the traffic (and public transport) into work (in the CBD). And seriously, with the route I can take to get to work, I do not need to break any laws on the ebike.
Also, I gave up my gym membership since I can get exercise riding instead. The ride also saves me an extra 1 hour per week day I was going to the gym.
I've calculated I'll "pay off" the cost (by not using public transport, not going to the gym and replacing some car trips) in about 2 or 3 years. That's just the "economics" and doesn't count the improvement in fitness (I hope). I suspect there's an environmental benefit as well, but I can't be sure given ebike construction/materials. Ignoring that part, there's obviously a benefit in replacing car, bus and maybe train use.
As for being a target for thieves - yeah, that's always been on the cards for almost any bike. Fortunately for my commuting, the office I'm currently in and the one we're moving to in the CBD, have secure parking where I can lock the bike. I've also got a bike lock that requires an angle grinder to get through.
So, yeah, definitely an investment, but one I consider worth it, but it's not going to be the same for everyone. It's just like some people spend a lot of money on their astro gear that others may consider super expensive.