I heard that none of the current Windows Phones will be eligible to receive any future updates...
then you heard wrong, there will be updates, you just wont get any WP8 hardware / kernal dependant updates.
up to a point this will stay true, i would "assume" based on absolutely no fact what so ever, that from release of WP8 MS will provide support for 7.x for a further 18 months, which would tidy over most users until the end of their current contracts.
remember folks, you own a phone that you selected and paid out for, bar bug fixes there really isnt and required level of support from MS at all.
it will be outdated sooner or later, but it wont suddenly stop working.
As for app developers, until WP7 market is small enough not to bother with they would be foolish to ignore it. If they can develop an app that is supported on Tango hardware then they would get a much bigger user base and thus potential sales then if they targeted it at WP8 only. MS has already said that porting to WP8 is already~80% done so its not like its a massive undertaking!
the other small thing is that as with all 1st gen devices, there will be much better ones around the corner so we'd do well to not move to WP8 for the sake of it just now anyway