The tl;dr verison: Spears (and other new weapons) could be implemented into the game with a bit of creativity in such a way that wouldn't be resource heavy for the developers and could turn a profit in the long-run.
Many call for "land spears" or even just additional weapons to become a reality, and many more yet vehemently oppose it on the basis of the following arguments:
Correct me if I'm wrong, but the meat of the argument against spears is argument #4. Nobody cares for the first three arguments in light of having interesting or cool new content overall and there aren't really any good arguments beyond the fourth one I listed.
If the interest was high enough, I think spears (and other weapons) actually could be introduced in a very economic and eventually lucrative way. Just think about it: currently underwater skins/animations/weapon skills are wasted assets. Nobody plays or enjoys UW content and Anet/the community knows very well that an UW-focused expansion (which is the only way that they could get us to play UW content) would NOT sell well.
Both of these problems could be fixed with a little bit of creativity: An expansion that focuses on the Deep Sea Dragon (aka. Steve). Just to be clear, it would NOT be an UW expansion. It wouldn't be marketed as an UW expansion and it wouldn't play like one either for the most part. The expansion might of course start in water or have some UW content and areas but could use a narrative or plot that focuses more on land.
Here are two examples of what I mean:
With the power of another other dead Elder Dragon slain (maybe unintentionally in the next expac or Living World episode), Steve has become enormously powerful. His power allows him to dry up/absorb most if not all of the bodies of water in the game. We then play the majority of the expac on land (in previously oceanic areas) with elite specializations focused on using UW weapons as land weapons.
Using his own power, or perhaps utilizing ley energy, Steve creates some anomalous terrain and maps involving strange water physics - think floating bodies of water, and water generally not obeying the laws of physics. This would involve a lot of land-based gameplay and perhaps some transitioning into/out of water. In this case, having elite specializations focused on using UW weapons on land would actually make sense from both a gameplay and narrative perspective.
Some creativity would have to be involved with the harpoon gun and the trident, but it's not outside the realm of possibility. For example, the harpoon gun could have some narrative that involves modifying the weapon to effectively turn it into a bowgun or repeater crossbow, and elite specializations that receive trident skills could focus on throwing the trident (lots of ground-targeted skills) as well as using the 3+ prongs of trident as ways of catching/parrying attacks to heavily differentiate it from both spear and staff.
If you think about it, it's viable as far as argument #4 goes.
Would balancing the new weapon skills be an issue? Not if they're attached to an elite spec. New skills must be created for elite specs with each expansion anyway, so having them attached to "new land weapons" in this manner wouldn't be an issue in this case.
I think if player interest was high enough, it's something that could be implemented to the benefit of both Anet and the player base.