The best defense in this game is a good offence. This isn't playable up to level 25, nor going to be viable after 25 for a long time. You're advocating playing overworld (not DFB) and arcs with 3 attacks until level 20 (Brawl, Boxing/Kick, Defensive Sweep), finally getting more attacks at levels 20-24 (assuming you're taking mez protection at level 10) and zero slotting in said attacks for damage until level 27 (at the soonest). Oh, and doing this without 2xp, so it's even longer to get to said additional attacks (and slotting for defense, no exp gained for surviving). And you're relying on a click power for your melee defense, a power which has a 5% failure rate, minimum.
Yes, theoretically, you can get to soft-cap Defenses by 25. But I'd love to see anyone play their Level 25 Hoverscrapper with 3 damaging attacks sharing a grand total of 5 slots (if you steal the EndReds from your toggles)... Let alone the climb to get there.
And, toons have minimal issues running unslotted to level 22 because the game boosts offense for lowbies (Beginner's Luck), and it returns to the point that dead mobs don't deal damage. You can sit in a herd at 45% to all, but if you can't hurt them, you're going to die... Streakbreaker works for mobs as well as players.
So, TL:DR, you've straw-manned the living FUCK out of this, to show that the powerset is bad because Ninjitsu is better than Super Reflexes... When none of your conclusions are true except for "it is possible to reach soft-cap on a Scrapper super-early" (22, since you don't need the EndRed, but do need level 25 IOs). Theoretically true, but definitely not viable.
And, because you tweaked my curiosity on this, here's a different take, that doesn't completely sacrifice offense, and WOULD be viable to play up "overworld with arcs, from 1 to 22, without XP boost, and spend all your merits selling converters and boosters." Also, it's Primary neutral, and come online at level 19 with 20 commons and 42% all defenses...
Frankly, this less highlights the OP/UP of a given defensive powerset, and more highlights just how ludicrously cost efficient to a build the 3% Defense all IO's are.