there have been mature experiences

I suppose someone will come along and argue that the singular should be used.

I would not agree with that. The author is speaking of the experience of small sum loans in Bangladesh, the experience of small sum loans in India, the experience of small sum loans in Indonesia, the experience of small sum loans in some African countries. It seems to me that the author is speaking of separate, distinct experiences--and therefore intentionally uses the plural.

If the author goes on to explain what the experience has been in each locale, that would confirm that the author is speaking of separate experiences.

I suppose someone [will] might come along and argue that the singular should be used but why they might do so is beyond me. 'experience' has a plural form, just like 'cat' or 'ball' does, which is meant to be used in situations where the plural is appropriate.