I don't think there is much more security because of the fact of having five routes. Fedex would simply ask your drivers to cover the routes, and rent vehicles, and even offer the drivers more than you paid them. Or they would ask other ISPs to cover the area, and those ISPs would probably jump at the chance, thinking they may be able to take over the area at no cost.
And fedex retains the right to cancel your agreement at any time if they decide to change the system. I think that the larger investment needed outweighs any benefit from being harder to replace because of having more routes. Fedex is fully prepared to take over for any contractor at any time, if necessary.
And I think fedex has more control over you, not less as an ISP. With the bigger investment, you have a lot more to lose, and that means you need to be more careful. Fedex may have given you a few choices over minor details, but if they don't like the choice you made- like uniforms, signage, etc., you may not find out until renewal comes around that someone who makes the choices they like takes over your area. The financial pressure on you to make fedex happy takes away any real freedom. You better do what they want, or else- and that is a big stick that doesn't really give you any freedom, except to say that you had the choice.
I agree that selling is a good idea if prices are inflated. Take the cash, and go to work for an ISP. Then you can invest that money in something like real estate, or even pay down your mortgage. If you make $300 a month less driving for an ISP, but save $400 a month on your mortgage, you've come out way ahead.