I think the concern some of us first person ‘die-hards’ have is that I can’t really see how it’s possible for ‘no cost.’
Surely it will take dev time/money to properly implement and produce some bugs which will need to be fixed. It also means that elements which may have worked from a first person only viewpoint now need to be reconsidered from third also - Not really relevant given the time period but as an example checking the time on your watch could be handled with a 3D watch model on the player’s wrist for a fully immersive FPS view, but would require a HUD element for third person (having a real ‘physical’ map would be another e.g.). Then someone might suggest just using the HUD for both as it’s quicker and already needed for third person anyway, or maybe a zoom in from over the shoulder which whilst the better option, would require more testing and so on… And so it begins.
Then there’s clipping issues etc, which even without third person supported interiors, would probably arise in some situations and need to be looked at and fixed. Some people will no doubt start to moan about not being able to fight from third person or go inside in third person - The “If you’re going to do it, do it right” argument and then there will be more time spent seeing whether that’s possible, etc, etc…
Another less than wonderful example: Personally I play Skyrim without a crosshair at all and it works brilliantly for archery (feels more like actual skill). This kind of stuff just isn’t possible from third person and whilst I’m not saying that’s what you’d want to do here, the whole “How does it work from third person” debate or worse “That’d be cool, but it doesn’t work from third person,” will no doubt come up each time a new mechanic’s mentioned.
A final example - It’s probably not easy getting the riding mechanic to look good from first person, but it’s something I personally really like the idea of. When you then have the ‘traditional’ option of third person for such tasks I fear that there will be the temptation of “Oh well, we have the third person option now, just get the first person camera to an OK state for the riding, most will probably use third person for that anyway.”
As you say TPV might be worth it financially if it is very easy to implement and will likely entice more players (= extra money for development), but I think it could quite easily open a can of worms if not careful and possibly limit what you can do going forward - When you know you only have to consider first person in all situations, your options are much more open. If you go first and third person then everything you do will have to be tested and considered in that mode as well (where applicable) and I’d be amazed if it doesn’t eat up a reasonable amount of time and money.