If that is true then they're already professional as far as I can see. That kind of money for a few hours work a week.Even if not full time they still earn enough that they could just umpire and do nothing else if they wanted. What would making them full time achieve?
According to this article: https://www.theage.com.au/sport/afl/pay-rise-for-afl-umpires-20121207-2b0qg.html
- Field Umpires - $150,000
- Goal Umpires - $60,000
- Boundary Umpires - $60,000
The only reason then why they keep making mistakes is because they are s**t at their job.