Its never just about how long the journey is. I'd agree that cost is a big factor but as you point out, not deliberately, human behaviour will drive peoples journey too. SE Metro is at peak capacity and many hate the crush. Offering them an alternative that might take them slightly out the way but get a seat or a comfortable journey will push people onto Crossrail. Same with their journey cross town. Some commuters really despise it. Chuck in reliability and you start to stack reasons why people will make a choice.
I also wonder what the impact of the timetable will be. Journey times are important but your arrival time at your destination more so.
If your walk from the station is more convenient then you may shift your journey accordingly.
Also, and I keep making this point, Crossrail isn't just about commuters.
Interesting point re: walking to stations and such. I used to work in Aldgate, and I would walk to Barnehurst (5 mins), get one of admittedly many trains to London Bridge (sometimes three trains in 20 minutes, so about a 10 minute maximum gap for the Victoria trains, journey time 30 minutes) and then walk to Aldgate. But, when it rained or snowed I'd then feel inclined to go to Cannon Street and take the Tube, as the No. 40 bus (or whatever it was) from London Bridge would be unreliable in traffic. Effectively, my journey time any which way was about 55 minutes.
If I was doing that now, I'd look to walk to the end of my road and drop onto a 301 bus that I know would come at certain times (e.g. 0723, 0733) with a journey time of 15 minutes maximum in morning traffic over West Heath. Assuming some walking time, spare time for any delays and getting on the train at Abbey Wood I'd now be looking at a journey time from door to Abbey Wood of 23 minutes or so; and then a train to Liverpool Street taking 17 minutes. I've now got 15 minutes in hand, and a 5 minute walk left. The trains will be newer, it will be fresh to go every 5 minutes from Abbey Wood at high peak and I'll have a seat. On the way home, if I do have to stand on Southeastern it's at least 30 minutes; but on Crossrail its 17 minutes and it's air-conditioned. Plenty going for Crossrail, and the 301 bus won't be a major issue on the way home as the bulk of traffic after midday is from 1515 - 1630 around Bexleyheath and Abbey Wood for school mobs. At about 6pm, it's not actually too bad.
But, my journey would not be the same for people just on the other side of the railway line near Barnehurst station itself. They would see the walk to the 301 bus go up from 4 minutes to 9 minutes, and they'd also find that on a rainy morning they'd rather sprint to Barnehurst and get on a train and have done with it. I'm sure some journeys won't even be repeated in future 5 days a week: it will be entirely dependent on a multitude of factors. Rain, amazingly, will clearly be one...