Princes Risborough doesn’t even have direct service to Birmingham and changing at Haddenham and Thame would make trip times probably half an hour slower than driving to the city centre. It’s not really comparable.
OK, let's compare Haddenham and Thame Looking at the ODM results and scaling up by a factor of 10 you might see:
110,000 people going to Birmingham (about 367 people a day or about 28 an hour)
7,650 people going to Manchester (about 26 people a day or about 2 an hour)
1,500 people going to Edinburgh (about 5 a day)
You'd likely get double that (as that's just one way), let's be generous and say the two way flow is rounded up to 120,000 and then 3 times this much. That's 100 passengers an hour, between 3 trains that's around 34 people who are traveling to/from places north of there either boarding or alighting from a train with a seated capacity of over 1,000.
Oh, and I forgot to say, by multiplying by 3 that's quite an uplift for just one way, arguably it should be the two way flow, in which case the numbers per train are likely to be below 20.
Haddenham and Thame has 600,000 currently to London.
First up is 610,000 as a two way flow to/from London, whilst i specifically only listed to London.
Second, if we scale that up by 10, the target also scales up by 10 to 6 million.
Bearing in mind that Farnborough Main has a town population of 65,000 and an urban area population of around 250,000 and has a within 15km a population of over 550,000 and even pre covid was seeing total passenger numbers of 3 million, that's quite a big ask.
Therefore, I'm happy to update what I so to be:
Yes, you may then see 6,000,000 heading to/from London, however given currently the Aylesbury stations as well as Haddenham and Thame (when all of them are combined) don't even see 1,000,000 to London, that's quite a significant change.
Also, interestingly, you cut the quote short (which can, and I'll let others decide if that's the case here, be something people do when they don't like what is being said in the context of the whole quote and is hoping that others don't remember the context) as I'd said:
Yes, you may then see 2,000,000 heading to/from London, however given currently the Aylesbury stations don't even see 300,000 to London, that's quite a significant change.
I'm happy to revise that to:
Yes, you may then see 2,000,000 heading to/from London, however given currently Haddenhamand Thame station don't even see 350,000 to London, that's quite a significant change.
Either way you look at it, that's still quite a significant change.
---------------------------------------------
In summary, to get to your 1 million (new*) passengers that's a significant change.
*if they aren't new passengers, then as others have pointed out, that's not new money and they don't count towards paying for the new infrastructure to allow a new station to be built.
As if the station isn't setup so through trains aren't against the platform edge, either you're slowing down every train as they pass through the station or you're installing platform edge doors and probably still slowing the trains down a bit as they pass the station.
Also that will need those trains which are calling at the station to stop fairly accurately (+/- 0.2, and even that means the doors are around 0.5m wider than the actual door) and so will need to have the sightly longer penalty for a station stop than if a train can have have a tolerance of +/-2m, as the last (say) 5m would need to be done at a slow walking pace to ensure that the stop was accurate.
As well as likely needing a reasonable extra gap behind them, so if the train was only 30 seconds late in departing wasn't then slowing the following trains.