Hindsight is always 20/20 but as FTX was growing and Alameda was losing money, he could have shut down Alameda before it went negative. Then, he could've focused solely on FTX, which was apparently worth 10 of billions.
Valuations don’t mean much until the money changes hands, typically with the asset holder losing.
Short answer is they're (their stock is) worth as much as people are willing to pay.