Because a Retribution can happily sit in the centre of an enemy fleet, trusting in its 4 shields to protect it, and put its full FP36 to good use. Whilst theoretically Invincible could do the same with its FP30, it wouldn't last long. Therefore Invincible is more likely to stand off, a role where good on-side firepower and range is crucial, and FP12 is not enough and the off-side FP15 is entirely wasted.
Somehow I don't think an enemy will just let you send a battleship in the middle of its lines. Unless you're opponent is kind enough to let you do it.
And so what if Invincible does match Retribution for firepower in certain situations? (which it doesn't.) That's the whole point of a battlecruiser! The firepower level is non-negotiable if this ship is to be included. It MUST be comparable to the mainline battleships.
The battlecruiser never was intended to be a mainline battleship even if it had comparable firepower. And again, FP12@60 cm is one step up in range compared to FP18@45 cm. You get an extra turn in shots at the expense of some firepower. Heck if you can get FP15@60 cm in an Invincible, then let's put that or even FP16 into the Retribution instead.
It originally did rival the Retribution (indeed outgunned it to the sides), but Retribution upped its game. Invincible needs to up its game also.
No, it does not, not with its current stats. If that's your issue, then let's just keep the 4th dorsal lance then.
Oh come on, it's not a complicated rule, so stop pretending that it is. Crits on 5+ with a +1 modifier only requires that you do the same thing as usual but count slightly different results. Rolling 2D6 per hit to see if you get a crit is an actual deviation from how things are usually gone. It takes 37 words to explain 2D6 Crits.
"The invincible class suffers criticals on a D6 roll of 5+ instead of 6+, and all rolls on the Critical Hit Chart have a +1 modifier."
26 words, crystal clear. This is the less complicated rule.
Did I mention anything about complicated? I am basically saying: "you're penalizing the Invincible twice with crits". That's 7 words that I hope you can understand.
As for which is tougher:
On a 10 hit ship with 2D6 standard crits, you'd expect 1.9 additional hits, assuming half of fire criticals are repaired before they do damage.
On a 12 hit ship with 5+ crits with +1 modifier, you'd expect 3.2 additional hits. So my proposal is still tougher than yours with 2 shields.
You seem to be missing the point of a true battlecruiser.
#1. As big as a battleship. 10 hits are NOT as big as a battleship. 12 hits are.
12 hits are not representative of just size. That is what you are missing or keep on ignoring. Your proposal appears tougher. However factor in the crits and how the battleship can easily blow up with a salvo. That's not tough.
#2. As powerful as a battleship. FP12/S3L/S6Torps are NOT as strong as a battleship.
The Retribution has had similar layout for the longest time and many people were saying it was good enough (though not me, I wanted FP18@60cm). Again, you keep ignoring this. Sorry, but with the exception of the torps, that is as strong as a battleship. And if you really want to make it as strong, then keep the 4th dorsal lance.
#4. Less well protected than a battleship. Having established #1, we're restricted on hits, whilst external armour is determined by IN Doctrine, so we're left with shields, turrets and special rules.
Exactly and while the battlecruiser is as big, it's certainly not as tough. To reiterate, 12 hits are not just representative of size. It's also representative of how tough a battleship is. That one needs to cause 6 hits for it to be considered crippled. That alone should prove to you that hits are primarily a function of toughness, secondary of size. The shields have been lowered. You don't want the armor profile changed and so the HP should be affected. 10 hits instead of the normal 12. Shields again should be 3 since the Invincible would have a battleship sized engine and should be able to afford the energy requirement esp with the lighter mass. The Battle Barge is 6+ all around and can still have 3 shields so I think it can do it even with the 4th lance.