I consider Tom Petty to be a vastly superior artist to Britney Spears. She has far greater commercial appeal however and her music is worth more today if you're selling a catalog. The same would factor in to selling the Dylan catalog. Dylan's music is not very popular with young people and will not be very popular in the future (see: Elvis, Cher, Neil Diamond, Clapton, Madonna, Whitney Houston, Rod Stewart, Chicago, etc; their music doesn't sell at all like it used to), although I'm sure it'll be redone endlessly. Do young people care about Simon & Garfunkel? Not in the least and they won't tomorrow either. That music will all fade by the year, relegated to a small fraction of the interest it once had, as the primary generation or two that listened to it die.
After 60 years Dylan has sold less than the Backstreet Boys. He produced relatively few hits compared to other renowned, popular artists. His style of music doesn't lend itself to vast commercial success. Someone will pay a premium to acquire his catalog because he's such a great artist and it's a large & influential catalog, however there is a practical limit to that.
You don't seem to be accounting for how widely covered Dylan material has historically been. There are plenty of Dylan songs that as far back as the 1960s brought more sales for others than for him. This has continued throughout his career. For example, one 1997 Dylan song was in the top 50 UK sales in 2010 and all told has been covered by more than 450 other artists. In the last five years, official releases of past Grateful Dead concerts have contained at least 30 Dylan songs. It's 600 Dylan songs and at least 10 times that in cover versions which continue earning money.
A quick google search shows tom petty has sold 80 million records and Britney Spears 100 million.
I think you are mis-gauging what happens to music over the course of decades. Bubble gum pop and similarly positioned types of hip-hop etc do very well when they are in fashion, but don’t earn as much consistently over the long haul.
Dylan’s catalogue could plausibly still earn 50 years from now.
There is also musical licensing, where a lot of money is made after the initial popularity. Technically, I have seen more bob dylan songs in commercials and movies than I have on the radio. Hey also has a ton of unreleased stuff in there two, which they probably can make a lot of money off of, especially/sadly when he dies.
After 60 years Dylan has sold less than the Backstreet Boys. He produced relatively few hits compared to other renowned, popular artists. His style of music doesn't lend itself to vast commercial success. Someone will pay a premium to acquire his catalog because he's such a great artist and it's a large & influential catalog, however there is a practical limit to that.