How do you know it will last 18 months?
..”it” being the present upturn in employment prospects.
The honest answer is absolutely nobody knows.
The present upturn might last several years, it might end next week. I’d treat manufacturers estimates with a massive pinch of salt (they have aircraft and training systems to sell). I’d also caution that another black swan event like 9/11 or serious events kicking off in the Far East could stop recruiting overnight.
If you regard spending on an ATPL as gambling rather than than investing, you won’t be disappointed.