There is a LOT of misinformation/FUD going on here that should be clarified:
#1 WP7 apps will work, unchanged and untouched by devs on all new WP8 devices. Melgross - MS did not say there would be work required for this. In fact, they said the exact opposite. Please, review the video where they talk about the compile in the cloud. There is absolutely no work required on the developers part - they don't even have to resubmit the app to the marketplace. Developers do have the option to modify their apps to take advantage of the new resolutions if they want - but, that is a choice not a requirement.
#2 Existing devices are not abandoned - they are getting the WP7.8 update at about the same time WP8 devices launch. Right now, we don't know what will be included in that beyond the new home screen - but, that's actually true about WP8 in general as well. The stuff that was shown off in the video was information about new platform changes and capabilities targeted at developers - we have no idea what other consumer features are being added to WP8 and which of those are going to make into the 7.8 update for legacy devices. Further, I haven't statements that indicate these legacy devices will not get any future updates beyond 7.8. It's been speculated for a while, that wp7 will remain in play as the emerging market strategy for low end devices. So, until there is an official statement one way or the other - it's all conjecture, and dare I say FUD.
#3 Developers who are working on WP7 do not need to learn new languages to work with WP8. Silverlight is a subset of WPF - and both of them use an xml markup language called XAML to describe the UI. On the backend - C#/VB.NET are still the recommended options for non-game development. So, the primary learning is really around the new api more than anything, and that's true with any platform update.
So, here's the reality - speaking as a wp7 developer - WP7 will continue to be supported for sometime by developers, just as previous versions of windows are still supported. Why? Because, if I don't need a new wp8 api or feature for my app - why would I not target wp7? That would give me the widest audience with no work. I suspect that the market place will work the same way it did during the transition from wp7 to 7.5 - that a developer can package both apps together, and the user will be presented with the one appropriate for their device. In other words, there won't be a lot of work to provide an app compatible with wp7.8 and another version (remember, they will likely share almost all the same source code) that is compatible with wp8. That part is of course speculation - MS hasn't revealed the whole dev story yet. But, based on the past I think this will be likely. I think the main area where users will probably most left out is in game development - there will most likely be games that are not going to be available on wp7.8 - but, to be honest, the hardware wouldn't be able to support high end games anyway. But, again if I was a game dev - and my game didn't require the high end features and native code, I would still target 7.8 for now.