These are still useful on a micro four-thirds body, just not in the way you might expect. With a speed booster, you reverse most of the magnification and throw a huge amount of light on those small sensors. A Nikon 50mm AF-D on a .71x speed booster and an m43 body is a disturbingly fast setup at ~70mm equivalent field of view.
All that said, the state-of-the-art fast primes for the m43 systems have left the classic 50/1.4 in the dust. The performance of the Olympus 25mm f/1.2 is utterly amazing. The only quibble with the lens is it's huge.
Modern lenses will always benefit from modern materials, design and manufacturing techniques. That said the benefits of M43 as a format are shrinking with the advent of small compact FF cameras. Your 25mm f/1.2 lens is equivalent (from a field-of-view and depth-of-field standpoint) to a full-frame 50mm f/2.4 lens or thereabouts.
You can get very compact 35mm bodies now, but lenses tended to grow in recent years to cope with the growing pixel counts. So overall a mFT kit will be significant smaller than 35mm unless you talk about a Leica M :)
The Olympus 25/1.2 is a stunning lens, but certainly large for a mFT lens. it is still slightly smaller than a Leica 50/2 L-mount lens.
Lens size and weight changes due to many aspects - number of extra corrective/aspherical/low dispersion elements, the _equivalent_ aperture, the number of focusing motors, whether the lens has optical image stabilization, whether it is weather sealed, whether it is made out of plastic or metal, etc, etc.
Its not easy to just isolate for sensor size. But _in general_, you can make small full-frame lenses which will be equivalent to micro four thirds lenses. The problem is that they won't sell. Today, few people want f/2.8, f/4, f/5.6 primes for full-frame. Most people shooting full-frame want F/1.2, F/1.8 or F/1.4 primes.
The size of the m43 kit was never the benefit, to me. The first thing I did was add a grip because the Olympus body was too small. Then I added another grip so I can hold it the other way. Now it's as big as any 35mm I owned.
Fair enough, but I'd wager most articles written about the benefits of the M43 format do bring up the size and weight savings compared to full frame systems.
To me, the smaller sensor in M43 does have a few unique benefits over FF.
* faster readout (less rolling-shutter)
* less power-consumption and therefore less heat-generation (longer record-time limits, more power for computational photography)
* less inertial mass for the sensor & assembly (better sensor-stabilization for hand-held video)
* higher wafer yield in manufacturing (hopefully lower cost , if economies-of-scale allow for it)
All that said, the state-of-the-art fast primes for the m43 systems have left the classic 50/1.4 in the dust. The performance of the Olympus 25mm f/1.2 is utterly amazing. The only quibble with the lens is it's huge.