I think the reason for the decline is simple - the number of professional users is shrinking. Large banks have shrunk their trading desks and the number of independent shops is shrinking too [1]. Couple that with the increased weight of quantitative strategies, which need a market data provider rather than a terminal, and you can see why the Bloomberg Terminal is in decline.

[1]: https://www.valuewalk.com/wp-content/uploads/2018/06/Biggest...