3. What size map do you play? The bigger it gets, the longer even blitz wars take. I typically play huge maps, which may color my views a bit. Aside from that, you're ignoring other methods of waging war. Do you ever put an early choke on an enemy? Furthermore, you're ignoring the main benefit of keeping control here: not avoiding the rebuilding of workboats (that's trivial in most cases), but being able to keep working those tiles, not matter what.
4. The point is that skillful use of a navy increases your relative production on an ongoing basis. If you build no fleet, the AI doesn't have to keep pouring hammers into his, and your shiny cannons are now facing his new shiny cavalry, the hammers for which could have gone into another frigate for you to sink.
3. Usually standard sized maps (continents, hemispheres, fractal or terra). Im ignoring other methods of war because theyve proven to be less effective, a war of attrition generally seems to hurt me more then the opponent. Im horrible at open field fights since the colleteral damage is so important for me in fights and the ai usually sends in a few small stacks instead of 1 or 2 big ones. In attrition wars my lands get pillaged and I suffer from war weariness alot, sure it hurts him too but im not going into war to hurt myself, i want become stronger by war.
Working boats only provide extra food, while food is nice this isnt needed much during modern wartime (it is when whipping alot), hammers are way more important. I can live with a few citys losing 1 or 2 pop points during war if it means the war is easier.
Ive used the warrior/archer choke a few times and it feels kindda lame, its on the edge of exploiting a bug and it isnt that good either on monarch (my difficulty level), a simple warrior rush with 6 warriors might cost alot more hammers then a 3 warrior choke initially but you odnt get the reward instantly (a new capital) and warriors tend to die.
4. Its only changing the relative production during wartime, and the wartime is way too short when blitzing to make it worthwhile.
Some random numbers to back-up my point:
Spendings using my strat:
My spendings: If you dont have a navy youll spend 0

on a navy, youll lose a few workboats, lets assume 5. Thats 150

.
Enemys spendings: The enemy usually has around 8 frigates on the ocean, thats 720

.
The enemy spends 720 - 150 = 570

more.
Spendings using your strat while having the same tech level:
Your spendings: Youll need 8 frigates to kill the enemys 8 frigates + around 8 to be sure your able to kill them all and keep some extras, to kill the frigates the enmy is building, thats 16 frigates. Which is 1440

, lets assume your also able to destroy 5 workboats, youll have to rebuild these after youve won so thats another 150

. Totalling 1590

.
The enemy has has 8 frigates at the start and manages to build another 5 during the war thats 13 frigates or 1170

.
The enemy spends 1190 - 1590 = -400

more, or 400

less
Having approx the same techs and using some weird numbers that means the total hammer difference is 970

in favor of my strat.
The numbers could have been better and your left with some frigates but you should get the point. In addition, if you lose 1

and your direct enemy loses 1

you should feel bad since your not dealing with just 1 enemy, you might be equal with your direct enemy but your losing against all others which is bad, and your not spending much without navy while you are with no matter how much

it costs the enemy.
Obviously the difference is much less when your using subs, but i dont have accurate numbers on those since i never use them

Feel free to correct me if it makes such a major difference that it suddenly tips the scale alot in your strats favor.