There has been pressure from some to increase the microlight weight limit even since I started flying in 1990. Remember then it was only 395kg!
Whilst it makes no sense to me that the weight limit of some aircraft in the UK can be increased considerably by minor mods such as addition of an electric fuel pump, the pilot training requirements are still less onerous for microlights than for Group A. It is still possible to obtain a microlight licence (albeit with operational limitations} with as little as 15 hours instruction, and have these limitations removed with as little as 25 hours under instruction. Furthermore the training is usually cheaper per hour and on more modern aircraft and engines than at a typical Group A training school.
So, be careful what you wish for. Increasing the microlight weight further may well make microlight licences (and the first foot on the ladder) more expensive to obtain and maintain.
If you want to fly heavier (and often more complex) types you can upgrade to NPPL (SSEA) or LAPL or even a PPL.
PS I have no axe to grind. I have not instructed in microlights for over 20 years and currently fly LAA types on a LAPL.