Your guess is as good as mine, I guess. I just would like to hear some logic. I don’t think band compatibility was just a foundation principle for the series 7.I think I disagree. They said that one of their design goals with Series 7 was to maintain band comparability. Therefore it will be something that will stay with us for a long time (maybe ever). Going from Series 3 > 4 > 7 was always a continuation/evolution so they kind of had to be compatible. With Series 8 and this new „Pro“ version they can do whatever they want because technically all the old bands will stay compatible with Series 8 (just not with this new size). Nobody will complain (except all these sensational YouTubers of course with their usual bs). But suddenly Apple can do a lot more with this new adapter/size. Remember all the patents about smart bands and sensors in the band? That might not come with S8 but it may lay the groundwork and it may not be possible with the current band adapter size.
Also: another band size also means another set of SKUs that Apple can sell. It might not be their primary reasons to introduce it but they will also gladly take the money. They’re the most profitable company for a reason.
Your supposition that this new watch that is barely bigger somehow must break with the entire line to achieve ruggedness and battery life feels extremely random.
Gurman says it’s an evolution of the current design. I don’t think the current band connector is losing sales to Garmin or incompatible with digital interfaces.