Compare one-time license versus annual software cost.
A perpetual licence is rarely a single payment: annual maintenance for updates and support usually runs at a fifth of the licence price. Totalling licence plus maintenance against the subscription over the same period is the fair comparison, and the break-even year is where the licence overtakes the subscription. Below that horizon the subscription is genuinely cheaper, not merely easier.
Licence versus subscription
Licence total = licence price + maintenance x (years - 1), where maintenance = licence price x maintenance share/100; subscription total = annual subscription x years
Because without it you stop receiving updates, and eventually compatibility. Comparing a bare licence against a subscription that includes updates is not a like-for-like comparison.
Set the maintenance share to zero, but be realistic about how many years the frozen version stays usable — that is the real cost of skipping it.
Subscriptions keep cash flow flat and updates automatic; licences reward long, stable use. The break-even year is the number that decides it for your horizon.