« With the exception of Buddhists, all of the world’s major religious groups are poised for at least some growth in absolute numbers in the coming decades. The global Buddhist population is expected to be fairly stable because of low fertility rates and aging populations in countries such as China, Thailand and Japan. »
In fact, for the buddhist population to be stable when fertility rates in Japan are definitely not compensating for the ageing population, this means Buddhism is spreading (partly via 'conversions') to other parts of the world. And although buddhists will constitute a smaller share of the global population, they might be better spread out in order to get the Dharma heard. Moreover, Buddhism is among the religions best suited for a constructive dialogue with the "non-affiliated", as it certainly accepts a secular form too.
Overall, there's inter-faith work to do, as called by the Vatican (e.g. enews.buddhistdoor.com/en/news/d/56977) and little reason to fret. This is a 'problem' only as long as we're trapped in "us vs. them" thoughts. And if one is worried about influence, then the reasonable conclusion is that buddhists should strive to work 'more' with Muslims, looking for commonalities, for shared intentions, etc. rather than 'less' or 'against'.
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The Future of World Religions: Population Growth Projections, 2010-2050