As my daughter might say, "Well, duh!" Of course ARM can't run legacy apps, just because it's running Windows. We're talking machine code incompatibility here.
On phones, MS "solved" this issue by tacking a CE, Mobile, and then Phone on the end of the moniker to indicate that it was a different OS. That wouldn't work in the case of tablets, where you're supposed to be running full applications.
The only solution seems to be running a virtual machine, as Apple did post-PowerPC, at the cost of slowing performance. You might not notice that on legacy apps anyway.
Games? Forget it. Emulation would make them too slow and the various bits get written in optimized assembler to goose speed even more. The graphics rendering might need to be redone for tablet graphics "cards" as well. It's probably more work to rewrite and recompile it all than it would be worth until (if) the ARM tablets take a substantial market-share.