Que sera seria - What will be WOULD be
Solution 1:
Taking a stab at answering your question, I had to consult a few grammar guides. It doesn't necessarily seem to me that #2 is incorrect usage of would in certain contexts.
The context of your question denotes a particular amount of uncertainty. That probability of the fish being putrid shifts the tense the longer the fish has been out of the fridge. As a stand-alone sentence, I don't think it's completely accurate, but with the added uncertainty, it could be.
Per Swan's Practical English Usage:
We do not normally use modal verbs to say that situations definitely exist or that particular events have definitely happened. We use them, for example, to talk about things which we expect, which are or are not possible, which we think are necessary, which we want to happen. which we are not sure about, which tend to happen, or which have not happened.
So the question isn't about whether the fish is or isn't putrid, it's about how certain we are that it is/isn't.
I'm imagining a situation where the context could be:
Oh! I think I left the fish out on the counter. Do you think it might be putrid if it's only been on the counter for a few hours?
I don't know. Considering how long the fish has been out on the counter, I think the fish would (probably) be putrid (by now).
Consider the following cases to see if these work for you:
(From OP) Considering how long it's been out of the fridge, I think the fish would be putrid.
- Considering how long it's been out of the fridge, I think the fish could/might/should be putrid.
- Considering how long it's been out of the fridge, I think the fish would have gone putrid.
- Considering how long it's been out of the fridge, I think the fish would probably be putrid.
- Considering how long it's been out of the fridge, I think the fish would probably be putrid by now.
- Considering how long it's been out of the fridge, the fish would probably be putrid by now.
This imposes a lot of stipulations on your original sentence which might not accurately describe the situation you're after, but I think this is the way I'd believe it to be usable.