I made $3k in 16 attempts to launch my SaaS
indiehackers.com
indiehackers.com
Your lifetime pricing is about the same as 6 months at the monthly rate. If everyone is choosing that option, its likely the lifetime price is too low, your monthly price is too high, or both.
Last thing you want is an income time bomb in 3 years where your most active and loyal customers no longer need to pay.
Maybe an alternative could be to offer free months subscription and a tshirt for every successful recommendation
Grandfathering can also be rewarded with a higher referral fee (as long as referrals are active, they can discount permanently).
It takes some planning up front with account, feature and billing management but can be worthwhile in general for flexibility.
While fewer than expected people might activate and use a lifetime deal, the ones that do can be a fun bunch support wise.
Asking lifetime customers may not provide a full picture though.