Defender on 7 was not active protection and the OS isn't receiving security updates either. That's a very risky combo.
Exactly on 7 it's not antivirus as on 8-10, is antimalware only, but it's active protection, resident works with ridiculous resources waste and fast. I have some typical captured virus for playing and it detect good. It continues updating definitions today.
About OS updates... well people like me, you? or other who did know RTB-ISDN, Win 3x, 9x, XP or IRC nukes with modems without NAT-firewall protection... it's minor issue