Let's talk about how consumer misunderstanding perpetuates a con started by camera manufacturers.
As we know, most digital cameras have sensors which are smaller than 35mm film.
This leads to a change in your field of view for any given focal length. A 50mm lens mounted on a camera with a 2x "crop factor", will have the same field of view as a 100mm lens mounted on a "Full Frame" or 35mm camera.
This is generally well understood but a lot of people have a subtle but important misunderstanding here.
The crop factor changes your field of view - but crucially - and contrary to most camera marketing materials - it does not change your actual focal length.
A 50mm lens is a 50mm lens, whether you mount it on a full frame body, an APS-C 1.5x crop body, or a Micro Four Thirds (MFT) 2x crop camera.
A 50mm lens mounted to a MFT body does not suddenly begin offering the flattering (for portraits) optical compression that an actual 100mm lens would.
Take an example of the Olympus 40-150mm F2.8 Pro lens. The advertising blurb for such a lens would generally run something like, "You're getting an 80-300mm (Full frame equivalent) lens with a bright F2.8 aperture, at a fraction of the size, weight and cost."
This is only a half truth. What are we actually getting?
A 40-150mm lens. (Focal length is focal length)
A full frame equivalent field of view of 80-300mm
F2.8 light gathering capability
The bokeh of an 80-300 F5.6 lens
These are not the same things. The use case might be similar in terms of field of view and exposure, however:
Full frame lenses have to cover a much larger image circle. This makes them more difficult to manufacture and it goes without saying that they need to be larger and heavier. They also offer more optical compression and a shallower depth of field when used on a full frame body.
So to market a MFT 40-150 F2.8 as being a direct alternative to a 70-200 F2.8 full frame lens, and on that basis charge a very similar amount of money is misleading at best.
The different systems are just different. An analogy - a small family does not need a large 4x4 to go away on holiday. They could equally well use a medium sized family car. That does not make the two cars equivalent. Nor does it mean that the manufacturers could justify charging the same for them.
In the same way, full frame is just different to APS-C or MFT. They just happen to have overlapping areas of competence.
So much fuss is made of equivalence. Search the forums and you'll find hundreds of enthusiastic discussions, arguing back and forward about the myriad nuances of DSLR Full Frame vs. DSLR Crop vs. Full Frame Mirrorless vs. Mirrorless Crop.
In the end, all the systems have something to offer and the question is which features are important to you?
However, for manufacturers to market something as being equivalent to something else, when it blatantly is not, is a con.