I think the reason why some (certainly not all) of those lenses deliver ugh performance is pretty clear: high photosite density in a small imaging area. Optical limitations—in particular, with M lenses, the oblique angles of incidence issue—are exposed.
You can add micro lenses and a host of other reasons to the mix. I don't know why legacy lenses don't work – they, often, just don't. Also, can you tell me why you get good results with the A7r but bad ones with the simple A7 ? High photosite density in a small imaging area, is surely not the (only) reason. I've asked the question before but no-one, so far, has answered that conundrum.
This is true assuming one views buying photo gear as an investment. I realize many people do. I don't. When it comes to cameras & formats I'm a serial polygamist. I love to try different stuff!
And that's great, I have 'no problem' with that, it's you, and those like you, that make this forum an interesting place to exchange views on gear. Without your feedback we'd be in fanboy territory. In contrast to you, I'm a gear 'minimalist' - I look for quality and simplicity. Simplicity, being one of the many reasons I bought the Fuji. To each his own.
I didn't diss either Olympus or m43 in my post, and it had nothing to do with 'investment' for long term gain. On the contrary, Olympus has produced great cameras and optics – they've got a great DNA. Not so sure about their past ethics, but that's another topic. SONY is now their largest single shareholder so I'm hopeful that, contrary to the NYT article above, they'll also survive the predicted meltdown.
I was pointing out, that for a different set of 'requirements' (see above) , there was ' still a decision to be made '. If you want legacy capability, it's SONY at $2,200. If you want m43, it's Olympus at a minimum system cost of about $5,000 plus. A pro will be able to write that off with depreciation and tax deductions. An amateur will have to pony up. That's all.