UI isn't a fashion show. I'd much rather have a UI that looks older but is comfortable to use than something trendy.
I quickly found a new client, but not one I love. If thunderbird does get a UI overhaul I'd give them another chance, and it sounds like they're moving towards a UI that I at the very least would prefer.
Asking as a NYer who would consider living there if the car-free life wouldn't be too painful
Biking is not great. I did it a lot, but I'm comfortable riding through heavy traffic on steep hills, if you're not, probably would be hard to rely on.
Finally, I'm big into climbing/mountaineering and lack of a car was never a big problem. I'd rent one the few times I had too (way cheaper than parking and insurance), but otherwise could usually carpool with someone else who was going. Sorta' puts you on someone else's schedule, but if you need a belay partner anyway...