eckhard
What you say is true, but I think it's belaboring the point. If the OP is asking about Vx/Vy type stuff, then I think they're still trying to figure out how the performance-limited ceiling works; and talking about hitting a Mach ceiling instead, only introduces more confusion at this point. It's a more basic question.
In figuring out why Vx and Vy converge, I'd advise ignoring Mach and imagining a lower performance airplane that will climb until excess thrust and power reach zero.