what if you change number or lose your phone?
You are given some backup codes when you enable 2FA which can be used if you lose your phone, lost access to that phone number, or don't have signal. Each code only works once.
You can also enable a fallback option. For paypal I an authenticator app as the main option with SMS authentication as a fallback. Also, I use Authy as the authentication app, which can run on more than one device. You should never need to contact customer support about the login.
Generally if you lose the backup codes and access to all 2FA methods you lose access to the account however (not checked paypal's policy but I know this is the case with several other sites I use with 2fa)
Honestly, it's very beneficial from a security point of view and well worth it for users. It's also well worth it for paypal because it removes most "i didn't buy this, someone hacked my account" type cases that I imagine they have to deal with every day. To scam/hack someone with 2FA enabled you not only need to steal their login, but you need to be on the phone speaking with them, or clone their sim card, or be physically with them, which makes it a time consuming process.