A coldfire would be the greatest way to soup up a classic Amiga, although you couldn't use PPC software (which is mostly the stuff you need the power for). It would still be great for everything else, though.
The argument that rages though is that lots of people would consider a coldfire an alternative to a new AmigaONE/Pegasos system and not buy one, dragging back and holding the Amiga community firmly in the 90s. Coders would still have to support legacy such horribly slow Amiga IDE, AGA, and all the Amiga bottlenecks. People on the new systems would get less support as people stuck to the old ones - at least that's what some seem to think. I'm not so sure.
I take both views. I'm happy to see people buy coldfires if they want them, it is their choice after all. But those who do buy them should *not* then harry coders who don't release 68k apps they can use 100% of the time, or whine that apps don't support AGA or Paula any more.