To me, TOTP means 'Top Of The Pops', I had to Google your acronym! How is Grandma Alice going to be able to manage these extra steps? - It's already taken over 20 years to convince people that 'Password1' is not a good password.
“Scan this barcode then enter the six digit number from the app; we’ll sometimes ask for the number when you log in” isn’t particularly onerous - 1Password for example will insert your one time pass along with your password so in some cases there isn’t even an extra step to log in. I’d be more worried about people losing/wiping phones and getting locked out of their logins - who needs those backup codes, right?
If someone gets your vault password and can unlock your phone, you’re toast, but SMS as a second factor is then also compromised so what usable (since this thread started as trying to sell MFA to lay people) options do you have (other than maybe a Yubikey)?
1: https://caniuse.com/#feat=webauthn 2: https://caniuse.com/#feat=u2f
After 1Password introduced MFA (TOTP) support, it has been used widely in organizations in shared vaults so multiple people can share critical logins that use MFA. This of course means that if your 1Password account is compromised it's game over.