The pay to win games must incur massive upfront costs to have such huge plane inventories.
Not sure about that. Some of them have a bunch of versions of the same plane. Here's just the Spitfire in one of those games including a floatplane spitfire:
Supermarine Spitfire Family (32 Models)
Great Britain: Spitfire Mk Ia, Spitfire Mk IIa, Spitfire Mk.IIa Venture I, Spitfire Mk IIb, Spitfire Mk Vb/trop, Spitfire Mk Vb, Spitfire Mk Vc/trop, Spitfire Mk Vc, Spitfire Mk Vb (Float), Spitfire F Mk IX, Spitfire F Mk IXc, Plagis' Spitfire LF Mk IXc, Spitfire LF Mk IX, Spitfire F Mk XVI, Spitfire F Mk XIVc, Spitfire F Mk XIVe, Prendergast's Spitfire FR Mk XIVe, Spitfire F Mk XVIIIe, Spitfire F Mk 22, Spitfire F Mk 24, Seafire LF Mk.III, Seafire F Mk XVII, Seafire FR 47
United States: Spitfire LF Mk IXc
Soviet Union: Spitfire Mk IXc
Italy: Spitfire Mk Vb/trop
France: Spitfire LF Mk IXc, Seafire LF Mk.III
Israel: Spitfire Mk IXc, Spitfire Mk.IX (CW), Weizman's Spitfire LF Mk.IXe, Spitfire FR Mk XIVe