The rules can be applied consistently - there is no need to complicate things. If an asset is issued with a freeware license and also with a payware license they are two different items. You can distribute the freeware one but not the payware one: that they might contain similar or identical parts doesn't matter. Whether or not the original creator is still supporting the asset, is still participating in the forum, is contactable, is still using Trainz or is even still alive doesn't matter: the original conditions apply.
Any additional conditions applied to DLS content would simply discourage contributions. If a content creator wants to include a waiver in their permissions they can. That's the sort of flexibility in licensing that needs to be maintained.
I think, and assume, you are using the two terms "freeware" and "Payware" as a simple comparison arguement, and NOT in fact conflating "freeware" with "public domain", so, please do not take this as personally directed at you, Sailor Dan...
But with that said, it needs to be clarified for those who are not legal-Rights savy: "Freeware" DOES NOT automatically give the "consumer" or "user" the Right to "redistribute" the "Freeware" asset, in any itteration, be it "original form", a "derivitive work", or simply a "repainted" version of an otherwise non-modified copy.
-"Freeware" is defined as being FREE for a user to use, or aquire, from the original source, for an indefinate period of time. "Freeware" does NOT give the User any other Rights, without other explicit written permissions listed in the Asset's EULA.
--In other words, generally speaking of the Definition and inheirent Rights there-of, "freeware" and "payware" are equal in all aspects, except that, the user had to pay a fee to aquire the "payware" asset.
"Public Domain" is the only classification of assets, in-which the original Author gives up all their Rights, and the USER may do as they wish with the asset. An good example of Public Domain assets, are those of Dave Snow, where he explicitly says that the assets are in the Public Domain as of this posting:
https://forums.auran.com/trainz/showthread.php?158331-Dave-Snow-Assets-now-in-Public-Domain