Almost every NBA player can play 2 positions. Booker can play the 4/5 as well. Bench players that can play 3 positions at a below-average level aren't as valuable as a bench player that can play 1 position at an average level.
We destroy all our depth? We destroy our versatility? What???
We'd have Lin, Crabbe, Davis, and Collins off the bench. Our bench gets better. Davis and Collins can play both PF and C, which you seem to value. Lin can play PG and SG, and Crabbe can play SG and SF. We could add a valuable vet. PJ Tucker would've been awesome, he signed for $8M a year. Patrick Patterson would be awesome as well, he'll make less than $10M a year. Or we could trade Davis and open up $17M to sign a PF like James Johnson (for between $12M and $15M). Johnson would be the perfect PF for us (and is a SF/PF combo). There's tons of options.
Say we start Booker. A bench of Lin, Crabbe, Johnson, and Collins gives us a PG/SG, a SG/SF, a SF/PF, and a PF/C. Position versatility...
Oh, but we need to retain are shitty role players because they can play multiple positions shittily... Lol.
What were our most common lineups?
Vonleh, Turner, Aminu were all part of them. Were they not?
Why were they part of them?
Vonleh could play 4/5 and switch on to some 3s.
Aminu could guard 2-4, but ended up guarding 1-5.
Turner could guard 1-3.
Jeremy Lin is a piss poor defender (but I'd actually like this part of the trade) because I like his game. Jeremy Lin overlaps with CJ and Damian, unlike ET, because he isn't as versatile, which greatly limits our lineups right there. Unless you plan on playing ball dominant Jeremy Lin with our ball dominant back court and being undersized at SF if you want to replace ETs versatility.
Thus, you just limited our depth and versatility quite significantly just by removing Evan Turner and his skillset. This doesn't even touch on the fact that no one we acquire replace Aminu or Vonleh, which makes us lose 2 possibly SF, 2 possible PFs, a C and a PG because those 3 players are capable of replicating the skillset of essentially six players (and not doing it as poorly as you say). Neither Davis or Collins have the skillset of Aminu or Vonleh, which is hard to replace. Collins we haven't seen at all so it's hard to say more there.
How does Jeremy Lin change any of that? What options does he give us that we don't already have? He also plays at our most crowded position, PG and SG since CJ and Dame aren't off the court much. He also will compete with Crabbe for Backup SG minutes when one of Dame/CJ is on the court being that he is too small for SF.
James Johnson could get up to 20mm/year and we have NO chance at him. PJ Tucker got 11/year and took the MLE with Houston because of His good friend, Chris Paul. We will see on Patrick Patterson.
So we get Lin, Booker and PJ tucker for Vonleh, ET, Aminu, Meyers and a 1st round pick? Talk about getting fleeced in a trade?
Dame/CJ/Crabbe/Collins/Nurk with
Vonleh, ET, Aminu
OR
Dame / CJ / Crabbe / Collins / Nurk
Lin / Tucker / Booker
Not to mention the first round pick we randomly give away.
And we still have ED and Harkless, who are better than OR nearly equivalent to Tucker and Booker.
I'd rather retain our assets than trade assets for older, similar producing players that have little room for improvement, in order to create limited cap space for a player who historically averages single digit points and rebounds and limits our rotations in a significant way, while the best player we receive in return replicates a skillset we already have and minimizes lineup flexibility and would receive limited minutes.
Better than accepting a shitty trade.