Definitely disagree there for two main reasons. First, people were hungry for 360 games back when Resistance came out, going exclusive on an expensive console killed both their sales and momentum. Second, being PS3 only means a huge mass of game players, namely PC and 360 shooter fans, are all but ignoring the franchise, and will continue to ignore it once part two comes out. This ultimately limits both sales as well as word of mouth and hence wide scale appeal. When people talk about top tier shooters, Resistance almost never comes up, not because it's a bad game, but because very few people can actually play it. This will continue to hurt them as long as they stay PS3 exclusive, even more so now since the PS3 is doing better in markets that (if you believe the forums) care little about shooters.
I don't think it would have sold like COD4 if it were multi platform, but it's not unreasonable to think that they could have shifted an additional 1 to 2 million units just by being on 360.
There is a very easy, yet simple solution to the problem those "PC and 360 shooter fans" are suffering from:
They need to buy a Sony Playstation 3.
Once they do that they will be able to pick up Resistance FoM and later Resistance 2 and even later KillZone 2.
Also the real reason why Resistance "almost never comes up" in so called gamers talk about "top tier shooters" is really due to the gaming media's coverage of the game and the nature of FPS games on Playstation consoles.
Back when Halo CE and XBox 1 came out, alot of the buzz was centered around Halo being the only game worth playing yet this word did not really come from word of mouth but rather the gaming media.
EGM is one of those examples, they kept on mentioning how much fun they had with "system linking" and how they just kept on playing system linking as part of how they gave the game its huge 10 upon release.
Now think about that, the XBox 1 is barely out, then Halo gets released and EGM had 8+ XBox consoles for system linking and its no surprise that the average gamer who can barely afford to buy one XBox 1 (back then) would evangelize to his friends the word of buying more consoles and game copies to enjoy a feature not found on the competing console due to technological limitations back then.
Its also not surprising how the gaming media kept on mentioning Halo and by mentioning, I mean free coverage, as in word of mouth and it was not limited to a single print magazine performing these practices.
Insomniac can go cross plat if they wish to do so, I would prefer them to be Playstation devs though but this talk that Resistance "is getting ignored because it is not multiplatform" is BS at best.
Its like everyone forgets all the things that happened with Halo 1 and how after boasting 2 million copies sold with no XBox Live as a factor, it basically set up the media and gamer frenzy that ensued with Halo 2's release and the existance of the XBox 360 as being the vessel to hook expectant halo and console FPS fans into a new more expensive console for the purpose of getting that game and then PC games making the profit due to the nature of both PC and X360 games being based on the Direct X API.
While CoD, MoH, Halo, UT, etc are established FPSs, Resistance is barely 2 years old and it will really be up to Resistance 2 and how the gaming media covers that game that is going to make or break that franchise.
That said another thing that held back Resistance is pure ignorance and the first two levels of the game don't help either, I am talking about how there is this perception that Resistance FoM is somehow a WWII shooter and not a "Alternate History Sci-Fi" shooter it really is.
The other thing is gamer apathy in comparing the game to Halo or a game that copied or ripped off halo somehow, including history twisting with some X360 owners actually believing that Resistance ripped off the halo bubble shield.
At least to me so far, Resistance as a franchise on the PS3 is going through the same growing pains Halo went through, so far given their time in months after release and prior to the sequel they have both reached a similar 2 million milestone and so far at least for those players there is a guarranteed 2 million sales for Resistance 2.
The rest is up to the Gamer media and new PS3 owners of witch there will be many more than last year by the end of this year.