Are we really arguing about this still? Time to pull out the big guns - actual population figures from printed sources:
"More than 100,000" (1e, 1987,
Cyclopedia of the Realms from the Gray Box Set, p. 87)
"Rarely falls below 122,000 beings" (1e, 1987,
Waterdeep and the North, p. 12)
"122,000 sentients" (2e, 1994,
Adventurers' Guide to the City from the
City of Splendors box set, p. 4)
"132,661" for the city and "1,347,840" for the entire region controlled by Waterdeep (3e, 2001,
Forgotten Realms Campaign Setting, p 178)
"132,661" (3e, 2005,
City of Spendors: Waterdeep, p. 6)
There is no population given in the 4e FRCS, or in the 5e SCAG or
Dragon Heist.
So, where is this 2,000,000 coming from? As far as I can tell, only from
Realms - Waterdeep | Dungeons & Dragons. So, what does that "up to two million" actually entail? Given the descriptions of the city in both 4e and 5e products, the size and architecture of the city hasn't changed all that much, certainly not so much to entail over
fifteen times the population repeatedly reported in previous editions. Granted, the Field Ward is new, and crowded, but, again, no description of it could possibly be interpreted that it could house 1.5 million extra people in a few acres; at best it likely adds around 10 to 20k more to the population. So, again, the 2,000,000? The 3e FRCS answers this -
it's the population of the region as a whole. In the century since that population figure, the area north to Amphail and east to the Dessarin River has added a few 100k, which isn't an unreasonable increase for a century, even with multiple magical catastrophes (Waterdeep, being at least minimally more stable compared to just about everywhere else, probably drew its fair share of refugees, above and beyond natural population growth).
tl;dr: The city housed 120 - 130k previously; there isn't any sign of new, Sharn-like, towers; the population of the region as whole previously was 1.4 - 1.5 million; so the 2 million must refer to the current population of the region as a whole as well.