Investment banks are toasting a banner year by paying their dealmakers more than at any point since the pre-2008 financial crisis boom. But investors are worried about when the party will stop.

The battle for top dealmakers — which led to huge pay demands from bankers switching jobs — pushed compensation costs to levels not seen in at least a decade at Goldman Sachs, JPMorgan and Morgan Stanley last year, which hold the top three spots in the investment banking league tables.