A better gun avalable from day 1, yep agreed, but you are making the assumption that these things are driven by purely Military Requirements and common sense ... As students of these things, we all know these two points are way down the list!
Other things like political interference and protecting the national industrial base take preeminence.
In the
UK
, Companies like BAE Systems run like an all consuming Cartel and drain the UK Defence budget.
Look at the US decision to go with the
M14
and ditch the Fal back in the 1950's, again all to do with commercial lobbying...
Should the UK have just procured and licence manufactured the M16A2 to replace the L1A1, probably, but that's far to easy, no fun to be had there...
If such an M16A2 procurement had taken place in 1985, you can bet those in the corridors of power would have found a way of making it late into service and cost three times as much anyway!