The problem going by big general eras is that they're so big. TRO Succession Wars has units that were extinct for half the Succession Wars. It also includes units that weren't introduced until the Succession Wars were nearly over.
The Hatchetman was introduced in 3023 so it's available in 3025. It isn't available in for most of the Succession Wars. And yes, the fluff in TRO:Succession Wars says when the Hatchetman was introduced but it being in book called "Succession Wars" gives the impression that it is available thorough the Succession Wars. The Talos was used up until the early Succession Wars so it also wouldn't be available for just about all of the Succession Wars.
Going by year though you know that the units are available. Players would still know what era it is. A reprint of TRO:3025 could easily have "Succession Wars" printed on the cover. Better yet it could have Fourth Succession War era put on it so new players have an even better idea of when to use it.
Going by year also makes it easier to back fill in all unit types. Star League era mechs can be put in TROs dated during the Star League. Reunification era units can be put in TROs dated for the Reunification War. For example the Mk1 LAMs could be in TRO date that fits their fluff, like 2705 or something.
So to be clear, I do not have TRO Succession Wars so I have not seen what is inside or made as part of the OOC introduction but I would imagine it has the same as many of the current books which is to explain the eras and I think offer links to the website & MUL. The mech entries may also have the icons for the 5 Eras that would be covered in the book- Reunification War, Star League, Early Succession Wars, Late Succession Wars-LosTech, and Late Succession Wars- Recovery.
The new player is not going to care that the era is that big- they are going to care about 'OMG, so many mechs in here!' Also complaining about mechs that are extinct for most of the Succession Wars, like that Talos, is a big strawman argument since all it takes if they are playing any sort of game where the fluff matters is- 'found it in dusty warehouse under a label AgroMech parts' and BOOM its not extinct for them in that game.
Using 'year' TROs does not let you know what units are available- the Savage Coyote was built in '59 (in FMWC) but was not in TRO 3060, it was in TRO 3067. Same TRO had the Hellfire (appeared FMCC), which was built in '58 . . . which means it was theoretically missing from 2 TROs. We also had the Cronus, which had a version that was out in the early 3000s, and had a L2 tech version but did not 'appear' in earlier TROs. TROs have NEVER been strictly 'this is what was made between the last year of the previous TRO and now.' IF a new player digs in enough to want to get into the story fluff, then the book is going to point them to the website which will lead to the MUL and they can then pick up the timeline-
IF they want.
The beauty of TRO Succession Wars as a starting point for new players is it provides a bundle of mechs that they can then play on a relatively equal footing- BV. They do not need to try to track down the 3 different books and entries in 4 or more to play.
As for a large number of TROs in print, a 3O+ year old game with thousands of units is going to have a lot of TROs. They should be available to players. Yes there's PDF but ebay and used book stores shouldn't be the only way to get print editions.
TRO Succession Wars and its follow on Clan Invasion along with IMO expected other 3 I mentioned do exactly that . . . it puts a omnibus TRO in print, that can be kept in print- just like the Beginner & GoAC Box- so that the foundational products can be kept in print easily. It makes a LOT more business sense, both from financial and beginner friendly, to have a single TRO covering a era instead of 4 or 5 that by your suggestion a company would have to try to keep in print. It may not be what you associate with the BT IP, but its a better business approach that is newbie friendly.
The last sentence of my post you quoted is sort of the point, you keep the information flow simple and something they can relate to . . . you may understand the explanation in the previous sentence rattling off all the intricacies and TRO dates but often times that sort of answer is enough to make someone look for a simpler game.