Plenty of shade? Maybe just a generally cooler area in your yard?

It's rhizomatic which helps ID but not a lot. That just tells you it's not tall fescue. If it's cool season it could be creeping red fescue, bent, or Kentucky bluegrass. If it's warm season it could be... anything really, but I doubt it's warm season because of your location.

KBG is extremely popular in that area. Most people will just do their entire yard in KBG.

Bent usually has a bluer leaf look to my eye.

10% confidence guess would be bermuda, a warm season grass, if that was a full sun area, but it usually does not pull up easily from a dethatcher because the roots are so strong and deep. Also you'd notice some straw coloration in your lawn most of the year up there in Philadelphia.

My high confidence guess is creeping red fescue because of your location, the fine leaf texture, and your description of how easily it came up off the soil. I have creeping red volunteer in part of my yard, usually in the beginning of the summer. It becomes really evident in the lawn and starts spreading but then immediately falls flat on its face once summer temperatures hit.

You can overseed tall fescue and get some plants going that will eventually shade it out, but to me tall fescue doesn't out compete anything unless you let it get pasture length, 6" +, and you seed every year. IMO that is not competing, that is you planting what you want.

You haven't destroyed your lawn. If it's creeping red it will creep right back in there by early summer next year. You have only pulled up what's on the surface so while not the most beautiful thing in the world, there is a great chance it will be back.
[Post edited by typed by ben at 09/13/2020 12:55PM]