I'm not a 740/740b user, but I fully understand the angst. It's clear that those products are being "sunsetted".
As I understand it, a major issue (maybe THE major issue) is that Microsoft has sunsetted Windows CE, which is the OS in these devices. But clearly another issue is the lack of horsepower in what is now a quite old hardware design.
iFly users that want a dedicated device need to seriously petitition Adventure Pilot to come up with a new device that has the horsepower, and runs an OS that will be supported into the forseeable future (e.g. Android). Then you can count on the form factor and brightness, as well as a hardware interface for NMEA output. I think that being a vendor that has a hardware product has probably served them well and they should consider continuing that. Hopefully it could happen, but even if it did, how long would that take before coming to fruition? Anyway, I doubt they'll do it. But there's no harm in asking.
IMHO I think the above posters are right -- you need to look somewhere else if you can't go with a tablet due to mount issues, brightness, or lack of native NMEA output. But having to learn new software, and losing the ability to run backup on a phone or tablet, or having it at home on your PC/laptop to do planning -- that's a hard pill.