With the changes to "degree of success" higher level spells mostly got the touch up that they do something if an opponent makes his save.
Not for poisons. They kept their really low DCs with doing nothing on a save.
For Fort saves, which are almost always the best monster saves, that's doubly important.
To give some numbers:
The level 13 poison has a staggering 80% chance to do nothing vs an (average) level 13 monster.
The level 10 poison has about 75% to do nothing vs level 10 monsters.
(with how monster saves scale, higher than 13 would mean even lower success rate, but there aren't injury poisons higher than level 13 either way, expect something like 15% success at levels 16+)
Stage 2+ is but a dream in everything except the most trivial (cr-3) encounters which you shouldn't use your 1 time use items either way.
For the exorbitant amount of gold required for said poisons, I would expect them to at least have a 60%+ chance to work as a one-off consumables.
So, 2 solutions imo:
A) deal damage even on successful saving throw, but end there.
Or
B) last 1h regardless of hits (so, at 20% chance of doing some damage, SOME hits would actually benefit from it)
