1) A precisation.
Amplify damage does NOT double damage, and Decrepify does NOT make summons make 50% more damage.
Amp is -100% to enemy physical resist, and Decrepify is -50% to enemy pysical resist.
So, assuming you make 1000 damage, if an enemy has 50% phyisical resist, with amp you'll make an effective damage of 1500, with decrepify you'll make 1000. If the enemy has 75% physical resist, you'll make 1250 with amp and 750 with decrepify.
And none of these numbers is the double or the 150% of 1000, related to the considered skill.
The thing you said is true only if an enemy has 0% physical resist. Enemies with no phys resist in hell are very rare.
2) The only curse that is useful for a summoning necro is Amplify damage. Decrepify has to be used only when there are tons of enemies like Moon lords with fanaticism aura or the xtra fast mod, cuz with that mods they are very dangerous, and slowing them can be necessary.
But in every other case, Amp > Decrepify.
3) Decrepify has horrible range when compared to Amp. Decrepify is very useful in PvP, but for PvP you've to use some different build to own, and not a summoner, unless you want to be owned.
4) As to "basically" immune monsters, all curses work at 1/5 of their real efficiency. So with an immune monster Amp will subtract 20% physical resist instead of 100%, and Decrepify will subtract only 10%.