I agree with OP`s musing that most offer toppers, ridiculous if the product works as advertised.
As to the whole topic of toppers and boosters on ceramic coatings, keeping in mind this is all theoretical cuz I ain`t no chemist but perhaps:
1. The elements that provide the durability and chemical/environmental protection of a base coating do not provide/are not compatible with the more entertaining aspects folks associate with a `good` LSP.
Things like slickness, exciting water behavior, beading, added gloss. In other words, the base coating provides the functional aspects, the booster the aesthetic aspects. Gyeon Mohs, Gtechniq CSL for example are some pretty tough customers but the visible water behavior is visually not that entertaining, hence the Skin topcoat in the Mohs-based Syncro kit and the `preferred` CSL +EXOV4 application combo.
On their own, Mohs and CSL are more hydrophillic as opposed to hydrophobic and that`s just not as visually entertaining as when topped with a more hydrophobic top layer. While Skin and EXO are `kinda coatings` like CarPro Gliss, Feynlab Topcoat and TAC Topcoat, the concept of topping a coating via dedicated `topcoat` or booster is, to me, kinda the same. The base coating, for whatever reason, can`t encompass all characteristics people recognize as `quality` behavior of an LSP.
2. Sacrificial layer or protecting the protection. Better to contaminate a top layer than the more expensive base coating.
3. Revenue generation. More products needed to maintain the base coating benefits, real or perceived. "You spent all this money on getting it coated, what`s a little bit more?" Kinda akin to changing oil every 3500 miles instead of following mfg recommendations of up to 10k intervals. Better safe than sorry?
4. Just following directions. Most mfgs reccomend some kinda maintenance topper, necessity of it being irrelevant. Again, better safe than sorry?
There`s plenty of well-respected coating aficionados who coat and let it ride...with excellent results so topping is definitely not necessary in all cases. May as well throw `personal preference` on the list as well as that is what drives a lot of this stuff, toppers, coatings, waxes, maintenance products, etc.
I dunno, really...interesting topic but at the end of the day, gotta do what works best *for you*. If that involves toppers/boosters then so be it. If not, no harm, no foul. Variety is the spice of life. There`s often little to do with a coated vehicle besides maintenance washes...the option to throw something else on top is a nice option if one is so inclined to occasionally change the appearance or performance of the base protection.