I think it's safe to say that Millie is becoming a rather popular character: she's French, she's the first female narrow gauge engine and she has a great personality. But one thing has bothered people: her whistle sound is the same as Lady's.
To those people, I ask this: does it even matter?! Since the beginning of the series, whistle sounds have been edited and recycled: Percy's whistle is just Edward's at a higher pitch, MANY diesels have used Diesel's horn sound (including Daisy and BoCo), Duck, Donald and Douglas have different tones of the same sound (well, they did until Sodor's Legend of the Lost Treasure), Neville used Oliver's Series 3 whistle sound, Peter Sam and Freddie used Emily's Series 7 whistle up until Series 12, Arthur has used Murdoch, Spencer AND Edward's whistle sounds in his life span, Bill and Ben's whistle sounds are similar, as are Bash and Dash's, Stepney had Skarloey's whistle in Series 5-7 while his whistle was similar to Thomas' in Series 12, Rosie's whistle is Thomas' with a higher pitch, Emily shared her whistle sound with Henry (or probably the other way around) until the back end of series 17, when she got a new one, and even Luke and Rheneas' whistles from Blue Mountain Mystery onwards seem similar, just a with different pitch.
So why should Millie be slaughtered for just continuing a "trend" that's been in the series since its inception? The whistle sound suits her if you ask me, and because she seems to be a permanent fixture in the series now, the whistle sound will now be used a lot more rather than just being forgotten about (as Lady will probably never appear again)!
Personally, I think Millie made a far better impression in 8 minutes 45 seconds than Lady did in an entire 90 minute film (or however long she spoke, I really don't give a damn about Shining Time Station: The Movie), not to mention the 20 second piece of fan service she received in Calling All Engines!.
Now, can people please stop moaning about this?! It's just a whistle sound for a character in a children's TV show; it's not the end of the world, it's nothing important (in the grand scheme of things). It's REALLY not worth moaning about something so menial!