Honestly I don't pay that much attention to the level, I just check the IVs and transfer everything under 90% unless it's something rare enough I would save it until a double candy event to transfer it. My calculation is that IVs are forever and level can always be fixed with dust at a future date. I used too keep >1000 CP rhyhorns because they could be cheaply spam evolved into good gym stuffers in the old meta.
I mostly get what you are saying, but my previous highest Steelix was 1780, now it is 1986. 2413 is my 4th highest Exeggutor, to me seeing that it got double grass means I'll keep it until I see a stardust tornado. Eevee evolves, bad attackers (Steelix); Exeggutors, even Golems I don't want to put dust into--some things are so common, I'd rather just have 6 of them that are ready for a raid. I have many legendaries, Dragonites, Tyranitars, and some useful 98% mons that need/want a lot of dust
Then I'm pretty sure any Steelix is junk, too. A 9/13/13 level 30 Exeggutor can hold a spot while I'm powering up a 15/14/15 BS/SoB, have it at level 32 now. I have 2 15 attack Confusion/Solar Beams at 2554 and 2510...I'll pokebattler check to see which does better against a generic Omastar boss, C/SoB with better attack or a significantly lesser BS/SoB***against a double water Omastar and no dodging the junk grass level 30 Exeggutor can win in 154 sec; 5 deaths. The better attacking C/SoBs that I would never TM nor trash are 160 sec
Attack IV fanatics can be that way, the way they play doesn't affect me...however, a higher defense value to go with a higher stamina value could make up for a 9, 10, or 11 attack in some cases...Also once a month somebody reports the difference between a 0 attack IV and a 15; then it is forgotten/ignored immediately and the IV engine of this game keeps on burning
A lvl30 15/15/4 Shuppet that I evolved for dex and got Sc/Sb - sure it is not perfect but it is fun to play with at times, so it can hang around.
Zero stardust for a lvl30 Sc/Sb sounds like a good deal.
...and a lvl30 12/12/15 Bb/X Pinsir. He can stay as he didn't cost anything for a nice dbl bug moveset. He cost me 2 pokeballs, and no dust.
I am sure there are others, but these are the latest couple I can rememeber.
Special mention to 2 eevees...a 100% and a 96% (15/15/13) both at lvl29 so they don't qualify for this thread.
Any "OK" Species I don't care about IVs, I only care about level 30
For example: Jynx
I would never bring any Jynx above level 30.
I would never use stardust even to bring a perfect hatched from 20 to 30.
But a 50% level 30 FrostBreath Avalanch Jynx I might use to quickly take out a dragonite. Or use it if Dragonite becomes a raid to get a little extra dps balls off the start. So it's a keeper.
This even applies to a few pretty good species that have low evolve costs. Eggy, Jolteon, Flareon, A level 30 Jolteon with 55% IVs works just fine against a Gary. A level 30 eggy with BS/Con SB I can throw into my 6 vs Suicine.
If it's something I plan on Maxing: It has to be Great IVs + Great Species.
If not, Level 30 is the way to go.