It is easy. The date and time on ticket office systems remains unaltered till you issue the tickets for this reason.
But the departure time from the next station would not be "the same", would it? Take a journey from Yorkshire to Cornwall, which would typically require 10+ splits to get the best price. As far as Bristol, you'd need to compare splitting at Sheffield, Chesterfield, Burton/Tamworth, Cheltenham, with splitting instead at Sheffield, Derby, Birmingham, Cheltenham, or possibly Chesterfield instead of Sheffield/Derby, and it may also be cheaper to split at Burton/Tamworth and Birmingham. Then from Bristol, you need to consider various other permutations. From each stop, you need to ensure you are looking at the XC train and not a following/preceding train by another operator, or the following stopping XC from Derby etc.
Not all splits involve advance tickets. Indeed, only a teeny tiny amount of ticket office sales involve advances, the vast majority are walk-up tickets for immediate travel
Depending on where you are (for example, if you're in the London area, the vast majority of journeys will be on Contactless or with Travelcards, so passengers would not be visiting a ticket office anyway), Advance fares account for a significant proportion of journeys.
By your own admission, passengers who are booking in advance and have particular requirements such as booking onto specific trains to meet their requirements, generally do
not visit ticket offices; this is surely indicative that people are not using ticket offices as an alternative to a website for complex / specific journey requirements.
On the rare occasions I'm in York ticket office (e.g. to use an RTV or get PlusBus), I agree with you that if there is someone else there, they are typically asking for something like "a return to Leeds", which would easily be purchasable from a TVM if they don't want to book online, so that puts into question any suggestion that ticket offices are there to get people the best possible deal for more complex journeys!
During the prolonged Island Line modernisation closure railway replacement buses ran from Ryde Esplanade to Havenstreet on the Isle of Wight Steam Railway as the Ryde to Shanklin RRBs couldn’t call at Smallbrook Junction. These were only advertised locally so didn’t appear on any internet based systems; tickets to/from Smallbrook Junction were required. As they weren't in the system timetable, online tickets couldn’t be purchased. Despite that I was able to get the ticket office at Rainham (Kent) to issue me the correct ticket, just on my say so. Well done to them.
Yes, it is often the case that ticket office purchases are enforced because of a failing by the rail industry to enable tickets for online purchase, for a wide variety of reasons, i.e. many of the transactions at ticket offices are made by people who have no other choice.
If many people genuinely preferred to use ticket offices than book online, the huge and incredibly busy York Travel Centre of the past would not have been reduced to a tiny office with hardly any patronage today.