If your phone has stopped getting security updates, then that is a big issue. Even if the phone is working fine. People are using their phones for banking, paying stuff, email, saving personal photos, etc. You dont want people hacking into your phone and an unpatched phone is an insecure phone.
Which is why samsung/google(and apple before them), have started offering 7 years of OS support. Modern phone hardware, especially flagship tier one, can last for a long time. Other than the battery degrading, the rest of the phone is still powerful enough for everything.
I think the Galaxy Store occasionally has some offers, like 50% discount on purchases and stuff. If you play gacha games, you can “save” a lot of money.