I used to think the same way till I got my 6"F8 dob with excellent optics. It absolutely blows away ANY 4 inch scope no matter how well refined it may be. BTW my 6" dob gives me views that are just as fine as my former 4" and 5" apos and is much easier to set up. Collimation is also extremely easy as is thermal equillibrium. No triplet Apo refractor will cool as quickly. The views in the 6 inch reflector are superior, in every way. Mechanicals are also quite excellent!
While my former large Apos were nice, they were not convenient to set up which is why I sold them and am no longer a fan of such scopes. YMMV.
My inexpensive, garden variety, Synta 6”f8 Dob also gives better views of anything that will fit within its 1.3 degree fov than my sensibly perfect 4” refractor. Although its optics could be improved, its mechanical systems are its weakest link, focuser, alt/az bearings, etc.
The three 6”f8 Dobs that I owned also cooled very quickly, but not faster than my 4” refractor. They didn’t set up any faster than my alt/az-mounted refractor, but they were easier to safely carry through a doorway fully assembled. I have not owned or compared them to 5” refractors, so I can’t comment from experience.
I have never owned one scope that comes closer to meeting all of my needs for an all around telescope than a 6”f8 Dob. Unfortunately, current 6”f8 Dobs are considered entry level scopes that are being continually cheapened (plastic focusers for example) to keep the price as low as possible. Unlike 8”f6 Dobs, there are no boutique manufacturers that I am aware of that offer premium versions.
Stock 6”f8 scopes are more difficult to upgrade because fewer high quality, drop-in, aftermarket parts are available. For example, a high quality, drop-in, 2-speed 1.25” focusers isn’t available; its OTA can be modified to accept a high quality 2” focuser, but this creates weight and balance problems. Correcting these problems pushes the scope’s total weight into 8”f6 territory, where it can’t compete optically.
Of course ATMs can build anything, but I prefer to purchase scopes rather than build them whenever possible. However, If I ever build a scope from scratch, it will probably be a high quality 6”f8 because I can’t buy one. What 6”f8 Dob do you have, and how did you aquire it?