It’s the training load (and expense) that the weapon system now includes. The calibre I don’t think is an issue (Army seem to have few concerms over introducing .300 Blackout for instance for the PDW) the ancillaries that make up the weapon system in it’s entirety are.When you say close-combatant rifles are becoming too specialised, is this in relation to a likely 6.8mm shift? 6.8mm rounds are heavier and if rounds do exploit the 80,000psi chamber pressure, I can see what you mean by non close-combatants requiring a smaller and easier calibre.
If this be the case - I'm a tad concerned about sustaining a BDE with two different small arms calibres. Non-standardisation and the impacts of it for logistics.
@Raven22 talked about it some time ago, the NFE, night aiming devices, suppressors, GLA, electronic architectures (rifle input control) to remotely control radio systems, light sources not on the weapon - potentially body cams in this new age and so on. In future possible links to UAV / UGV and so on only make it worse.
It’s not a rifle with iron sights any longer, it’s a specialised combatant system, that has to be tiered with the kit that is supplied depending on need or else trained on specifically and Army has found that non close-combatants (ie: special forces and infantry) need simpler systems as their training time needs to be focussed primarily on their corps role.
Then of course, as always there is a cost issue. Goodness knows what an MCX with all the fitted ‘fruit’ would cost to run, but certainly not cheap…