Well, as in many cases we are following our Commonwealth partners. Canada has had an all ranks, all Services, Regular and Reserve long service medal for nearly 70 years.
The Kiwis have an LSGC system based on our own but, according to the NZDF website, they introduced a long service medal for commissioned ranks back in the Eighties. Not a bad looking gong either. They even backdated the qualifying service from 1985 to nearly 8 years.
I think the Kiwi system is a fairly happy medium, albeit still fairly radical for the UK.