I'm not worried about "drone" technology taking over for cargo aircraft. That technology, if ever...is at least three decades away. Too many problems with security/safety and FAA certification alone will take 10 years.
"Amazon Flex", much like Uber/Lyft, will likely wind up no more profitable than traditional services when regulation finally arrives, and it will. Bezo's is just the latest EGO to make the mistake of thinking he knows something about the airline industry.
Groups like UAL, et al look good right now, but you have to understand that their business plan is no where NEAR as profitable as Fedex or UPS. Pax airlines operate on average on a 2% profit margin, while purple or brown typically are several times that.
Remember, overall your work life should be fun but it has to pay the bills.
In the end, go with what fits you best. Just remember to never feed....TROLLS.