[mst]

It's not like modular cameras would be a completely new idea... medium or large format, anyone?

[Klaus]

Back in the film days, many people were willing to pay for a booster or even viewfinder ...  
Today you can at least buy vertical grips.
External flash units are nothing else but customizations. External mics ...
You can buy external viewfinders for some of the EOS M cameras.

It's not as if you can't customize your camera these days.
In all seriousness - why shouldn't it be possible to make the viewfinder modular?

Of course, I don't know the market size.
However, I think that we all have our no-goes when it comes to deciding on a camera. And why shouldn't it be in the interest of camera makers to provide solutions for "easy" aspects.

Those boosters and alternate view finders were for the professional top model cameras, for professional photographers. They were high priced, low volume things for those who needed and could afford them. So.. Your point was?

♪♫ And isn't it ironic, don't you think ♩♬ (Alanis Morissette)...

"The (de-)attachable viewfinder is a bad joke in the year 2019."
Klaus from Optical Limits on the Canon EOS M6 mk II, August 20th 2019.

The viewfinder on the M6 IS a bad joke because it's not seamlessly integrated but merely attached to the hot shoe.
I have such a viewfinder for the M3 and it's just not a practical solution to the issue. It's hardly sturdy and just always in your way in your camera bag.

I was talking about something like this here, of course (also note that the hot shoe remains available unlike on the M6):

[JJ_SO]

It's not like modular cameras would be a completely new idea... medium or large format, anyone?

In the beginning, I read it like "I like to fill a checklist with stuff like sensor, battery, EVF, LCD, baby-seat, radio, nave-system and then I get the camera manufactured and printed with KLAUS-1". Big companies could finance this concept, but don't dare. Daring companies can't finance it.

Oh, and the Pentax LX-1 was such a huge success... blew all the Nikon F2, F3 and Canon F1 just out of the bags of press-photogs, right?

[Klaus]

The LX is certainly more comparable to the mirrorless size format than a F3/F4/F5, don't you think? ;-)

[mst]

Oh, and the Pentax LX-1 was such a huge success... blew all the Nikon F2, F3 and Canon F1 just out of the bags of press-photogs, right?

Well, it's easy to call a camera a fail looking backwards, the question is was it a fail because of its concept or because of the brand name printed on it (and as such being part of a bigger or in this case smaller ecosystem that maybe wasn't attractive enough).

Since you mention the F3 yourself: it had a modular design, too. 5 different viewfinders were available, combined with no less than 16 different focus screens, an optional motor drive, bigger film storage and different backs.

[Klaus]

Oh, and the Pentax LX-1 was such a huge success... blew all the Nikon F2, F3 and Canon F1 just out of the bags of press-photogs, right?

Well, it's easy to call a camera a fail looking backwards, the question is was it a fail because of its concept or because of the brand name printed on it (and such being part of a bigger or in this case smaller ecosystem that maybe wasn't attractive enough).

Since you mention the F3 yourself: it had a modular design, too. 5 different viewfinders were available, combined with no less than 16 different focus screens, an optional motor drive, bigger film storage and different backs.

q.e.d.

[JJ_SO]

Oh, and the Pentax LX-1 was such a huge success... blew all the Nikon F2, F3 and Canon F1 just out of the bags of press-photogs, right?

Well, it's easy to call a camera a fail looking backwards, the question is was it a fail because of its concept or because of the brand name printed on it (and such being part of a bigger or in this case smaller ecosystem that maybe wasn't attractive enough).

Since you mention the F3 yourself: it had a modular design, too. 5 different viewfinders were available, combined with no less than 16 different focus screens, an optional motor drive, bigger film storage and different backs.

q.e.d.

I was thinking about writing "the Pentax LX was the closest moment for me to step into the Pentax world" as I really liked the concept and some of the things Pentax simply did different - but then the explanation why I did not do it at the very end also came back: If modular, then why  stay with tiny 135 film? Costwise it was not that far away from a Mamiya 645 Super, and the Pentax had no interchangeable film magazines.

Neither had the F3. And it's backs are no substitute for a fully functional film magazine. Also, for standard hot shoe strobes you'd need an adapter. One downside of modules...

The Rolleiflex 2002 had them, but still was a 135 type and the whole modules lifted the price region close to MF, not to mention the limited lens choice of sort of normal Rollei lenses or sort of less normal, but steeply priced Zeiss glass. That's why I still think buying a well designed integrated body with all the features I need is better than legoing it together (and it was never seamless, Klaus). As AF these days happens on sensor level, I would be curious how interchangeable sensors would behave. My guess: Worse than integrated systems.