I'd like to see actual numbers. I'm not saying Solo is not the most populous mode, but with 400k copies sold (heard this, not stating it as fact) and a PVE group like Mobius only sitting at around 5-6k members (again, heard, not stating as fact), there remains A LOT of room for reality to be quite different than individual player perceptions (like mine and yours).
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Until there are actual numbers, then any discussion based on the merit of "the most populous mode" is conjecture and achieves nothing in these discussions.
Because it doesn't suit your views. How convenient...