Nobody is 'stealing' routes from anyone. This so-called 'regional' flying was set up my management and agreed to by labor when they signed contracts with concessionary scope language.

Most routes that are now flown by 'regional' airlines used to be flown by mainline pilots on mainline airplanes but the pilots voted in contracts allowing it to be flown on smaller airplanes, usually in exchange for larger pay scales on larger airplanes.

If mainline pilots want it back, reject any contract proposal that doesn't include a tightening of scope. Hard to put that toothpaste back into the tube though.