That surprises me. I don't know what Bexleyheath town centre was like before Bluewater, but today it looks to me like a largish, very pleasant, and thriving centre. I go there a couple of times a month and it's always busy in my experience. I'd say the problem from the point of view of trains is that the Bexleyheath line doesn't actually go anywhere near the town centre, so very few people are going to choose to use the train to go shopping there. It's the same problem at Eltham, and I'd say is a big part of the reason why the Woolwich line is busier: Woolwich Arsenal station is right in Woolwich town centre, and Greenwich station isn't that far from Greenwich centre.
I would disagree for two reasons: Firstly, pricing. If you're using Oyster pay-as-you-go, then getting a bus even for 10 minutes adds £1.50 for a 1-way journey, and £3 for a return journey. That's a huge disincentive for people to do a part-bus part-train journey. Similarly if you want to get to Lewisham by doing train to Greenwich then DLR, you'll wind up paying more on Oyster because you'll have to pay a combined tube/national rail fare instead of a pure national rail fare. That could of course be solved if TfL and the TOCs got their act together on integrated Oyster fares, but there's no sign of that happening. And even if they did, you'd have the problem that the poorer reliability of buses would make a lot of rail users reluctant to use them.
The other reason the direct Lewisham-Woolwich trains are useful is for people wanting onward connections at Lewisham. As an example, I often travel Abbey Wood-Nunhead. Changing at Lewisham, that's perfectly doable and reasonably convenient. but if I had to change at Greenwich as well then the journey starts to look much less convenient, and also becomes much more awkward for timing: The Lewisham-Nunhead trains only go every half hour, and if I have to factor in the uncertainty of how long I need to wait at Greenwich into my calculations of whether I'll make the connection at Lewisham, I'm going to have to add more padding to my journey. Lewisham has trains only half-hourly to a lot of destinations, so I imagine people going to a lot of other places would have similar issues if there were no Woolwich-Lewisham trains. Obviously not a disaster, but does show that those trains do serve a purpose.
