what happen if a post passed a mod's review but then reported as plagiarism the next day?
Then it'd be removed. Some things will inevitably slip through the net but it's about minimising the damage in the first place.
mods don't have enough time or appropriate tools to do those tasks (on all new posts)
More mods can be added to meet demand, but spam is something we should be dealing with anyway. At least this kills it before it has a chance to be seen or spread.
then the new trend of blaming mods will begin in meta forum 
furthermore they will start attacking mods credibility while keep spamming to overload mods' works
furthermore they will start attacking mods credibility while keep spamming to overload mods' works
Well we'll always be damned whatever we do, but I don't see many people complaining in Meta about their trashed or locked spam mega thread, do you? Plenty of new users come here and ask the same old things that get trashed on sight but there's not much complaints about that either. There will always be resistance and complaints about anything we try to do to minimise spam (see the one merit requirement for Juniors) but that doesn't mean we shouldn't be trying things, especially when we do still have a huge problem with spam and are likely to have for some time.