This has been raised before, ie. should N3V have a clearly defined product life-cycle for their DLC? I can understand them wanting to offer benefits to those who subscribe, but I'm not sure that works for stuff that's years old. There should be a critical path that leads to:
(a) beta and RC versions available to Trainz Plus beta subscribers [t-3 to 6 months]
(b) initial release to Trainz Plus (at discount) or Gold subscribers (free) [t=0]
(c) release to TRS19/22 owners (payware) [t+6 months]
(d) release to DLS (freeware) [t+5 years]
In this way you are incentivising each tier of your membership as well as ensuring that your product has an 'afterlife' that will last beyond the immediate release. Offering models only to subscribers - as well as creating a two-tier Trainz membership - risks models becoming very 'niche' and only known to a small fraction of the Trainz userbase. Also, of course, such models cannot be used in Sessions that are designed to be uploaded to the DLS for the whole community.
Of course, if N3V wish to fully embrace SAAS - which is their right - then the next version of Trainz should be subscriber-only and not available to buy and install.