Hi,
From my experience in talking with manufacturers about various multiple units, the issue is the amount of tooling that is required is restrictive, three different complete vehicle tooling's is the vague limit for most companies as I think the law of diminishing returns comes into play after that (and more significantly than say a Class 37 or 47)
A lot of multiple units have more than three different vehicle types in their consist, particularly the 9-12 car fixed units we see. Then you get into variations, even within a 'common' family of units means that the tooling costs become far higher than the standard loco, especially when lots of units come in limited liveries (for the moment).
Whilst I know there is some modern stuff on the way, I think we are running out of stuff that makes business sense!
The argument about units are needed to 'get kids into modelling' in my opinion is complete rubbish, kids are still coming into the hobby and will continue to do so, there is no evidence to suggest that the hobby is 'dying out' because of the lack of younger modellers (yes lots of shows and clubs are falling away due to lack of younger members, but that's because a lot of younger modellers are preferring not to join a club and have a presence online). Yes, lots of kids like modern units and the stuff they see, but there are also lots of kids who like 'pre-grouping' stuff or BR era stuff, so I don't necessarily think it is a limiting factor that there aren't that many modern units about.
Having said that, it is an area where manufacturers need to look at again, now that various manufactures are proposing more multiple units and the market is content with the purchase cost.
Simon