Great question; been thinking the same myself. Thing is, I may already have it! (E tu, KTMboy? Say it ain't so!)
I recently bought a KX/KDX200 hybrid, and thought it would be my retire bike. It'll do, but it doesn't have the grunt and snap my RMX250 does - essentially the "scare" factor that makes riding fun and at times challenge, so I'm going to keep them both until one day in the future when parts are no more, then reconsider. They stopped importing the 250 6 years ago (and development like 15!), and the KDX in the mid-90's. To hold on to some aging beasts sounds crazy to some, but I cannot ride either for all they are worth, so there's not much to gain in a trade up, although it's tempting.
It may be best to try different bikes out before the commitment. That way you have some idea if it's a keeper or not.
Have you considered the new CRF250F? I like the concept, maybe even electric start, but can get by without it. Although I believe you can get a better deal on the KTM250RFS E/XC and believe it's a great mount, I've read there are those who bought it then became disappointed in the power, close-ratio trans and being similar in weight and maintenance compared to the bigger bikes. A big-bore kit would certainly do no harm.