Thanks everyone for your thoughts and suggestions. I finally decided to keep my A6400. I looked through last year's photos and noticed that there was hardly any situation in which my camera wasn't fast enough.
The 6700 is certainly more comfortable to use with its new menu system and the touchscreen.
I also researched many review sites and found several comparison tools: in all of them high ISO noise was noticeably worse for the A6700 so I wasn't even sure if the new camera would be an upgrade at all in terms of image quality.
Maybe I'll reconsider once there are more affordable A6700s on the used market. Until then I will continue to enjoy my good old A6400.
I think you made the right decision. The A6400 with the Sigma 56 is incredible. Maybe there will be a successor to the A6400 this year.
I had Sigma 56, but it doesn't have image stabilisation. I took the camera with only the Sigma 56 to a wedding reception, as I got it just before the wedding, but focal length was too long for people around the table, pictures with subjects far away had too much noise on dim lights, or blurred with lower shutter speed, then videos were bad with no image stabilisation.
I'm surprised anyone would trust a reviewer much these days. Considering the click bait habits let alone their slanted review methods. According to Photon's measurements off the sensor, there is NO question there was an advancement in the newer 26mp sensor used on the A6700. Whether every user uses it in a manner to extract that remains questionable. DR ratings of 10.51 vs. 10.95 on the 6700 may not be a huge jump but it's not static. Low Light sensitivity is where it jumped from the 6400's 2470 rating to 2643. Low light EV sensitivity jumped from 9.63 to 9.72 once again not huge but nevertheless forward.
I get your not desiring to move into a new camera. I'll say this however. The focus system on the A6700 alone IS light years ahead of any previous generation. This may only mean something if you're shooting wildlife, birds and children moving around. The boost in the brightness and speed of the EVF is welcome to say the least. I find in daylight NO lag at all. And the electronic shutter has been moved big time forward with a speed that mostly avoids rolling shutter in photography I've found. The larger more ergonomic grip, more buttons, articulated screen etc.
I just don't care for people who diminish the technical advancements that HAVE been made to a product. Only if they don't take advantage of them. But denial of their existence isn't' dealing with real world usage for many of us.