Exactly, and the solution to this cannot be a suggestion. Players will simply ignore it. It needs to be a rule, one enforced via in-game systems. The zombie systems have to stop. As I said in my post, any kind of requirement to continue on that forces them to invest in these systems would dramatically improve the situation. I disagree with the other poster about increasing the distance because then people will just do the math on the cost of upgrading a system to get increased range vs brute forcing zombie systems. It can't be a choice. It has to be a hard wall, a requirement. You MUST, upgrade some systems to move forward, it is non-negotiable.
I'm totally fine not putting that requirement on each player. Something akin to needing a certain level of system within such a distance to colonize planets around it. This could mean any player could have the responsibility and people could work as a team. They could still chain outposts, but at least in this system they would occasionally stop to upgrade one so they could continue. It would make them think more about the path they take to their ultimate destination rather than shortest distance and dropping an outpost wherever. It would also incentivize other players to help certain systems that are upgrading so they can also move forward faster. People would be more likely to pool resources to get to an end goal, or to expand. If they couldn't move forward until a system needed to be upgraded, and a local system was at 50% of the requirement or something, that player would probably help so they could make the outpost they want. Currently there is ZERO reason to help randoms, so this kills two birds with one stone.
If they aren't going to fix this then I would honestly rather have them remove the distance restriction so there aren't zombie systems littering the galaxy. The entire point is to be an architect. To take pride in it. To make a system worth going to.
Whatever they do, they need to do it fast because players are moving at a ridiculous pace leaving these zombie systems in their wake.