I'd fight for a position which has a natural monopoly even if there's an incumbent in the space if the incumbent isn't particularly strong. If BTC becomes 100x bigger, MtGox's current position doesn't matter much. If BTC (or other cryptocurrency in general) doesn't get 100x bigger, it doesn't matter and both MtGox and a new competitor will fail/be irrelevant.
I suppose it all hinges on how 'particularly strong' you think the incumbent is. I think MtGox is quite strong: they've survived a number of trials-by-fire, they've grown with Bitcoin, they're the most-linked place in 'how to buy' guides, they're the default source of 'the price'. People who view 'the oldest' as 'the safest' (a popular and not bad folk heuristic) will always lean toward MtGox.
The transition to CoinLab management is a risk factor, but is likely invisible to most people. If it goes off without a hitch, the new links with silicon valley capital will mean any well-funded assault would be answered with a well-funded defense.
