Ultimately the A220 and E-2 series are going to be bought because the can service a small niche more efficiently than anything else can serve that niche. Unless there is another big recession that forces mainline pilots unions to yield up scope, these aircraft, at least those over 76 pax, will be flown at mainline. To the extent that these aircraft can increase the flying done by the public, we all benefit, because they'll become the entry level mainline equipment that will be filled - at least at the FO level - by new hires. There will still be communities where smaller jets - even 50 seaters - can still be economically competitive but increasing salaries and bonuses (and training costs) will decrease the current advantage regional's have in crew costs.
Basically, it isn't the A220 or E-2s, it will be the economics that determine the fate of the regional's.