The question seems less "Should pops be added?" and more "Can pops be added in such a way that maintains the integrity of an EU game?". We already have a very rudimentary pop-style system, and it's called development. While it is true that development =/= population, it is an abstract representation of the human presence in a province. Obscure Siberian wastelands have the minimum, while large urban centers such as Milan, Paris or Delhi have a large amount. It's thus inarguable that on some level, development is a partial representation of population.
With this in mind, I really cannot see how an improvement of that existing mechanic, whilst keeping to the skeleton of an EU game, is anything but ideal. EU5 should be, in many ways, an improved and streamlined version of its predecessor, revised by a new set of eyes and minds and also making use of modern capabilities. Considering that EU4 already flirts with the idea of population at constant intervals (look at how events interact with it, for example), what's wrong with going a step further and outright implementing it? There is nothing in the base concept of pops that necessitates micromanagement or over-complexity (never mind the fact that EU4 is a game practically defined by those things). Sure, if a bungled and poorly-conceptualized version of that feature is implemented, it's detrimental. The same goes for any other base mechanic in a grand strategy game. If we have faith that Paradox can continue to develop EU4 in such a way that it remains enjoyable (as apparently many anti-EU5'ers do), then surely we also have faith that they can implement a pops system, which they've done several times already in other titles, in a satisfactory way?
The arguments opposing it also seem to be from the perspective either of "This specific pops system in this specific game is bad" or "It sounds too complicated". Neither of those are valid, especially in a game that, like I mentioned earlier, is built upon complexity. While us experienced players may now look at EU4 as a rudimentary sandbox, it is in truth an extremely complicated game, so much so that it often intimidates many new players into seeking out the Civilization or Total War franchises instead. A little more complication, especially when replacing a very unintuitive and arbitrary system (development and mana), is not going to kill anyone.
Pops seem like a necessary step forward so that the EU title as a whole can grow into its own. EU4 is currently a game that represents some of the most pivotal turning points in human history, which exists practically during the entire age of colonization, which deals with advent of modern warfare, the transfer from feudal and estate-based governments into centralized government, sets the stage for the industrial revolution and deals with everything from schismatic religious warfare to pre-communist people's revolution and so much more, while also not even acknowledging the idea of human population. You can play an entire playthrough of EU4, conquer your entire continent and forge the most powerful military on the face of the earth, all while your capitol city doesn't grow by one single infant's worth of human life (hypothetically). Trade remains static and entirely based upon which country has the most manufacturies and who controls the best per-determined river estuaries, armies appear out of thin air based on arbitrary 'force limits' and how much money you have to throw around and economies are built entirely off of either the aforementioned static trade, static production or a static tax presence that, once again, remains unchanging for potentially hundreds of years unless you actively choose to develop or an event occurs for that specific place. This is a fundamental and glaring weakness that brings the entire game's potential down with absolutely zero reward, and yet this should be sacrificed in favor of... what? The devs continuing to build mission trees for gods-forsaken tags that a total of five people have played? So that EU5 can be a rehashed clone of EU4 but with better graphics? I'm not seeing a trade off, I'm just seeing a resilience to change from people who have apparently forgotten that once EU5 is published, EU4 will still exist and will still be able to be played. It is not written anywhere that EU4's strengths, diplomacy and war, have to be done-away-with or indeed at all diminished in favor of pops. In fact, especially regarding warfare, pops would be a net benefit by incredible margins.
TL;DR: Pops, if done right, have the potential to set EU5 up above its peers and remedy many of EU4's weaknesses, whilst also preserving and reinforcing its strengths. The argument against it boils down to "Stellaris's pop system is bad, therefor all pops systems are bad and should not even be considered", "It sounds hard, so we shouldn't bother" or "I want more provinces in Zimbabwe and some obscure Polynesian tag doesn't have a mission tree, therefor the entire next game should be delayed". In other words, there practically isn't an argument against it, at least not one that is being held in good faith.