I figured, I would try to figure out just how well you can do to defend yourself while teching, without sacrificing economy, assuming your opponent does a very strong rush.
The rush I decided to pick for this is the chaos cultist + chaos marine rush, which I think is generally accepted as being a good one. The build order for that rush, or my execution of it, anyway, is:
1. queue up heretic, and 3 cultists
2. make temple, heretic that gets made helps with temple
3. each cultist squad goes to cap an SP
4. as soon as temple is done, make chaos lord, then marines and marines again (DON'T queue them up right away)
5a. if you're going for pure rush power here, you will make 1 LP, and proceed to kill both your heretic to make squad cap room for the 2nd squad.
5b. if you're not, you can research the squad cap upgrade, and still have 200 in time to make the 2nd marine squad, you will however have less money for reinforcements - 150 cost for cap research, vs 50 cost of LP AND getting the extra +6 req.
6. when the 2nd chaos marine squad finishes, queue up several reinforcements for the marines, put all the squads on overwatch, and go attack the enemy, chaos lord leading the way.
In the tutorial on Fallen, this rush will just reach the enemy base at 3:50, regardless of whether you picked 5a or 5b, though 5a will give significantly more troops, but you won't have any heretics left. Note that this is about the fastest possible way to get 2 squads of CSM, CL, and 3 squads of cultists, as the temple is help-built continuously produces units.
So, the teching player has 3:50 to put up a defense that's good enough to hold this rush... and they'd better have a mobile force, and a good economy.
Here's what I settled on doing, again with Chaos:
1. queue up 2 cultists, heretic
2. heretic makes temple
3. cultists cap nearest SP's, when done, they proceed to the next nearest SP's
4. 2nd heretic makes power generator
5. whenever each heretic is done, make LP on the nearest captured SP's
6. make chaos lord
7. when first LP done, upgrade it and make 2nd generator
8. when 2nd LP is done (you get +50 req), CL should be done for a few seconds, and you'll be able to make marine squad.
9. upgrade 2nd LP
10. put up 1 minefield near each LP (NOT in front, rather off to the side and right next to it.
11. reinforce the marine squad.
At this point, you will have seen their attacking force (you have the cultists out in front, remember). You also have ~150/75 resources, just enough to start building a bolter turret on the side the enemy is attacking at, should you so desire (its not needed to hold, though). Oddly enough, the time when the last minefield you put down finishes is 3:47... which should be just before the enemy comes in.
So, what do you do? When your cultists spot the enemy, run them back to base. You have a Chaos Lord, a total of 10 cultists, maybe less if you lost a squad to the oncoming army, a full squad of marines, a minefield and an upgraded LP at each entrance, AND an economy good enough to overwatch every squad you have constantly. In the tests, the enemy rush gets torn up quite badly - the AOE mine damage to the cultists is quite effective, and their economy is far, far inferior - they don't even have ANY power income, and not enough requisition to constantly reinforce everything.
If the enemy microes the rush reasonably well, I suppose they could make a close battle out of it, but your superior income should put you over the top. If they choose to sit back and deny you more expansions, you can pump out an overwhelming force very quickly, again, due to your stronger economy. Whether you choose to tech to armory, defilers, PSM, or simply a mass of CSM, is up to you.
Of course, this is a contrived situation, as it does not address harassment. However, the defender isn't very vulnerable to harassment, due to having the upgraded LP's very early, and only relying for +12 of the income on other SPs. Also, if the attacker is doing the above chaos rush build, they will not have much time to harass at all. And the attacker can be harassed, too... I think this DOES address the question of whether, all things being equal, an all out rush can be defended against by a heavy tech build. At least in the Chaos vs Chaos scenario. Throw all the other races in there and it's going to get pretty... chaotic
Thank you for reading. Any thoughts, or constructive criticism, would be much appreciated.