It's not hard to write Hadoop map/reduce queries in other languages: mrjob for python is particularly nice.

It's also not relevant that SpiderMonkey is being replaced with V8.

Mongo's map/reduce is also just a toy, not at all comparable with Hadoop's and being deprecated in favour of the aggregation framework.

Also, there already are decent alternative map/reduce implementations. Disco is a good example, with a similar design to Hadoop.