Wanting to know how to swap an ac/5 for two heat sinks and a PPC, or SHS for DHS is what most people want to do, and pretty much all a beginner needs to know. I'm proposing TW2 has only the most basic of construction for a reason, and it isn't even to sell splatbooks.
You keep saying "basic construction", but you seem to be implying "of Battlemechs" and not of all of the units the new "Total Warfare" will contain rules for without saying that. I took your "basic construction" to indicate of everything that the book would have rules for. And it should if it's going to contain construction rules at all. Unfortunately, those aren't basic the more units one adds in.
The thing is, that is what
A Game of Armored Combat rulebook does as an introductory book. The next "Total Warfare" isn't going to occupy that same space any more than it does right now, if there will be one. I think that is something that a lot of people are making a mistake with their statements here.
I think it's very important for construction rules to be in the core rulebook. New players aren't always going to have access to Technical Readouts and Record Sheet volumes. Imagine if the D&D Player's Handbook didn't include rules to make characters, and instead told players to go visit a website to purchase some! Even "journeyman" players, who graduated from the AGoAC, Clan Invasion, and Mercenaries boxed sets, are going to expect an expanded design system. In strict marketing terms, it's a value add. TechManual made more sense when BattleTech was - bluntly - mostly selling to already-established customers, but the reach of the brand is far beyond its nadir in 2006.
You're about 10 years or more out of date, acting like this is the 90's. TROs? Record Sheet packs? Due to the efforts of several different groups, The Tech Manual is only necessary for a player to understand WHY they can or cannot do something in one of the several programs available for unit design.
Furthermore, if they have AGoAC, they already have basic 'Mech construction rules. The Tech Manual allows one to take that system in AGoAC and expand it to other units and systems.
The reason it was separated out becomes obvious when you look at how big the book is.
Let's look at how the Tech Manual is constructed, cutting out fluff:
Basics: 12 pages.
Battlemech: 16 pages.
Industrialmech: 12 pages.
Protomech: 10 pages.
Combat Vehicle: 16 pages.
Support Vehicle: 14 pages.
Conventional Infantry: 12 pages.
Battle Armor: 14 pages.
Aerospace: 20 pages.
Equipment descriptions and ratings: 74 pages.
C-Bill cost and availability: 28 pages.
Battle Value: 18 pages in book, or 23 pages in Errata.
Record Sheets: 17 pages.
End book Tables: 11 pages divided in Heavy Weapons (3 pages), Industrial (1 1/4 pages), Ammo (1 page), Battle Armor (2 1/4 pages), Conventional Infantry (3 1/2 pages in book, 8 pages in Errata), and Aerospace (1/2 page).
And while construction rules are useful, if not down-right important in the right sphere, they aren't needed to be brought to a game. Everything can (and should) be done before one shows up.
The advantage that
The Tech Manual brings in being separate is that you don't have to bring that extra weight of those pages you won't be using at a game. As it is, for an average night I bring TacOps Advanced Rules and Total Warfare. I like using The Battlemech Manual when it comes to Mech stuff because it is better organized, as well as having that nice "Misconceptions" section at the end. If I bring any other book it's TacOps: AU&E and Interstellar Ops (don't have the lighter Alternate Eras yet) for equipment not found in BMM like tube Artillery and Cannons or Tripods.
The only time I brought The Tech Manual in to the store was when I was literally teaching a course on 'Mech construction.