I'd say you're slightly misreading the point of the patent system; I'd argue the underlying "point" is what it says it is in the constitution -- to promote the progress of the useful arts + sciences -- which isn't exactly the same thing as "come up with ideas that people wouldn't otherwise come up with".

EG: one aspect of the patent system is that it promotes invention of work-around, "me-too" inventions to get around patent restrictions (eg: PNG vs. GIF, for a computer example).

I'd argue that's more often a bug than a feature: worthwhile inventions would happen anyways, and engineering-around-a-known-solution is far more often a deadweight loss than a benefit to the economy as a whole; there many others (mainly members of the patent bar) that'd argue that such work-arounds are a feature, as they promote "new invention" that otherwise wouldn't happen (since you'd just use the known-good solution).

I'm sort of assuming you usually see this "my way" on this issue; if you do, then framing it as "think of stuff that wouldn't be invented otherwise" doesn't give you much to stand on (as clearly most workarounds wouldn't be thought of without the patent system...).

Sticking with the promotion-of-progress language gives you a much firmer frame for your arguments.

Your framing also is making you overlook the importance of disclosure in the patent system; it's not an accident that a patent not only describes what it does (separate alumina from bauxite) but also how it works (supposedly in enough detail that someone else could implement the invention by reading the patent, though in practice there's a strong incentive to obfuscate that as much as you can get away with).

The argument here is that this promotes the progress of the useful arts and sciences as it makes the knowledge underlying a particular invention available essentially immediately -- as soon as the filer rushes to the patent office -- allowing work on derivative inventions to start immediately, thereby increasing the rate at which new ideas are come-up-with, etc.

So while on the one hand you're kind-of right -- patents that have lots of simultaneous inventors probably are too obvious to be useful -- you've not addressed the real thrust of the delay issue (which I admittedly could have made clearer):

- for "good" patents in your system there's an incentive to wait as long as possible (as the longer you wait the greater the odds your patent is valid); this'd be especially true for the ones that don't have much to worry about from co-filers b/c they're legit inventions

- this means that the rate of disclosure of the genuinely-novel inventions would be expected to go down, as ceteris paribus there's more incentive to delay filing and therefore delay disclosure of the underlying ideas

- so the calculation what is the effect on the rate of progress of the useful arts + sciences under your proposal is roughly ("increased progress due to lack of bogus patents no longer gumming up the works") - ("reduced rate of disclosure of truly novel inventions"), and imho the latter term would be quite substantial and would need more arguments to justify it

The simpler hack to get what you want is removing the presumption of validity (too lazy to check if I mentioned it already or not); this would change the patent-infringement workflow from:

- file lawsuit; patents assumed valid until (or if) defendant successfully challenges every relevant claim in every relevant patent

to:

- file lawsuit; filer must successfully "validate" each relevant claim in every relevant patent, and only then will proof of infringement imply damages are merited

...and this can be phased in in ways that'd not be crazily disruptive (eg: phased introduction and/or the presumption of (in)validity is on a per-area basis, so pharma is presumed valid but not software).