It's possible that some people will take the bus to Abbey Wood to catch Crossrail, rather than going via Lewisham onto the DLR, I don't know if this will have a major effect though
It does seem likely that some people who live between Bexleyheath and Abbey Wood who currently go to Bexleyheath to get a train there would swap to CrossRail. Bus links to Abbey Wood from the South are pretty atrocious at the moment, but I understand TfL are working on improving bus links to Abbey Wood in preparation for CrossRail, so I wouldn't be surprised if Bexleyheath-Abbey Wood bus journeys became easier. Also, there's anecdotal evidence of quite a few people driving and parking around the common at the top of Bostall Hill and walking from there to Abbey Wood. That's likely to increase.
However, overall, I can't see that having more than a very modest congestion-relief impact on trains through Bexleyheath, and it certainly won't be significant for the DLR at Lewisham. I would expect the main congestion-relief for the DLR to be on the Woolwich branch, since most people who currently board the DLR at Woolwich Arsenal will use CrossRail instead. I would expect a significant number of passengers along the corridor between Excel and Bank/Tower Gateway would swap to CrossRail too. But as others have pointed out, that'll all probably be more than offset by future growth in the DLR anyway.
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Not worth it with current traffic levels let alone when the thousands of homes + added traffic arrive. From Bexleyheath the only road is the narrow Knee Hill really. No chance of widening as through ancient woodland. There's other longer alternatives but busier and no time saving beyond SE to Lew then DLR.
Knee Hill will certainly get even busier. However, since parking opportunities at Abbey Wood are limited, I imagine relatively few people will drive down that road to get to the station. What you will see is more people parking around Bostall Heath or down New Road (already happens quite a lot even without CrossRail). Buses would use New Road, not Knee Hill, to get to Abbey Wood.