Your math manipulation is correct but misleading. Frantik is more correct on how it actually effects the game. More range power adds damage, that damage is used to take monsters to zero hit points. The percentage of the boost compared to your existing damage is irrelevant. If a monster has 100k hit points, adding more damage kills it quicker. The base damage is what matters, not what percentage it gave you compared to what you already did.
Looking at your 100k HP enemy:
If you have 1k DPS and add +2k, you kill it in 33s instead of 100s
If you have 10k DPS and add +2k, you kill it in 8.3s instead of 10s
If you have 50k DPS and add +2k, you kill it in 1.92s instead of 2s
There's an obvious difference there in how much adding flat damage does lol. You can also use this to judge your cc and defense evaluations - cc is a lot more valuable to the low DPS build (a tank in high skulls? Idk), but I personally would be picking survivability vs 0.08s faster kill time if my survivability was in question. Realistically it's going to be more granular, like figuring out if you can decrease the number of hits to kill an enemy mob - if they're dying in 5 hits, does adding X DPS allow me to kill them in 4 hits? If not, it's meaningless outside of bosses. And for bosses, it's a percentage again - "wow I kill this boss in 17 seconds instead of 18, saving me 1s per quest!"
I often hear the Self Defense vs. IAF for a shuri thrower. Always throwing in the "when questing" caveat. Choosing Self Defense is weak, it is a crutch. You are a ranged character. If you want to survive longer, kill the mobs faster. There are plenty of other sources to increase hit points without losing 10 RP with a potential 50 on top of that against bosses or high HP mobs. If you are ranged, your positioning is more important to your survivability than your hit points. If you keep dying, play better. It sounds mean or cliche, but in this circumstance, it is the correct response. Nerfing your damage for a couple hundred extra hit points makes no sense.
I mean, dump Con and go for it. Just get good, don't take damage. If you're fully stacking IAF against trash mobs, skill issue - you should be killing mobs faster than that. If you're only getting a couple hundred extra HP you need a better gaming chair.
If you're bringing a tank, your survivability is largely meaningless esp for questing. If you're not, then presumably there are sometimes enemies that can hit you (with spells, etc). If you're always surviving those, get more DPS. If you're never surviving, get more survivable. Most people are somewhere in the middle, where sometimes you die (generally to specific things like dot stacks, horrid wilting, etc etc). For those, it's tradeoffs. Being able to accurately assess the value of a tradeoff is important.
For people getting into R10 questing and raiding (the ones reading this thread, presumably) it's disingenuous to push damage as the always-correct choice. I have thrown thousands of rezzes in raids to people with more damage than survivability.
There are also plenty of players I would suggest Self Defense over IAF even for raiding to, many of whom I've run R10's and raids with. Health problems, aging, low attention span, IRL distractions - these are all factors that push the scales more towards survivability (although you get plenty of time to deal with IRL stuff as a soulstone, so YMMV).
DDO does have some minimum hit point expectations and some defensive expectations for every character. Generally these expectations can be geared for. It is very rare that you need to sacrifice damage in feats, enhancements, and filigrees to hit these expectations. But once your character is able to hit these expectations, do your job, put everything else into damage.
Sure, once you're not dying it's time to max damage - but there's no threshold of "if you have 250 PRR you never die" that covers all of DDO. Different players will have different expectations, and judging build/gearing/playstyle choices to fit those expectations is important.