There should be no issues. If an application works under MacOS 1.0 (not X) then it should also run fine under MacOSX10.7 what ever. Apple should not be breaking our tools. Same goes for the hardware side. The modern computer has the capability, the power, to do any necessary emulation. All old software all the way back should work on any new Apple hardware including using it on the iOS.
I do agree that Apple should try a bit harder with backwards compatibility; certainly I think that the latest versions of Safari, Xcode, iWork, and iCloud should run on Snow Leopard (the OS just isn't that old; the last update for it came a year ago today), and not supporting iOS 6 for the original iPad just sucks.
But Apple generally handles transitions extremely well. Think of the OS 9->OS X move, which was well executed with the Classic/Carbon/Cocoa strategy. Plus the PowerPC->Intel transition, which, save for Intuit products and maybe Microsoft and Adobe, end users really didn't need to worry about thanks to Rosetta and Universal Binaries. We're also currently in a transition between 32-bit and 64-bit, which Apple is handling deftly and quietly, also thanks to Universal/fat binaries. (When was the last time you had to worry about whether you should run a 32-bit or 64-bit app on your Mac? Probably never. These are things Windows users have to deal with.)
But sometimes, it's time to move on. Removing Rosetta from OS X saved a lot of time and effort for Apple; they no longer had to compile the entire operating system for three different platforms (ppc, i386, x86_64). Removing 32-bit support in Lion similarly saved Apple from having to target the kernel for two platforms (or three, if you include iOS) every build. I wish they had waited an extra year or two before getting rid of Rosetta, but such is life. It's Apple's decision, and other than Quicken, it hasn't been a huge deal for most users.
A good friend watched his ProTools recording software (only two years or so old) cease to function with Lion.