Well, first we need to define the meaning of the word 'failure'.
Now, losing to the Wii in terms of overall sales would not be enough to consider the PS3 a failure. After all, both target different demographics, aside from the 'hardcore' which buys multiple systems anyway.
That is, of course, not assuming a DS-level success for the Wii, in which case PS3 will fade into insignificance. The same happened with the PSP. On a flat rate, it's selling well, but DS rubs it's 100K+ sales in on a weekly basis.
If the Wii really takes off, or the PS3 price starts to hold it back (also with the distinct possibility that gamers might be getting tired of the same games), then Sony will be forced to take action.
But here's the issue - Sony retains no control whatsover over Mothra - and their mind-control chips implanted in Gojira have varied results, if we're to put it in the most positive light. However, as long as they get Gojira to Tokyo, he's likely to rampage, with or without direct commands issued by Kutaragi. (Stringer would never allow himself to be implicated in such a process.)
That said, if Sony were to unleash Gojira on Japan, it is unlikely that Mothra would show up - unless one faces a threat Gojira alone will have trouble handling. (King Ghidorah perhaps, but latest reliable reports have placed him under heavy sedation in Yamauchi's basement.)
Mothra, as we know, is largely a benevolent monster (in comparison to the others, anyway). She wouldn't rampage even if she showed up, unless it played a role in some larger scheme. That said, with Gojira rampaging, it wouldn't matter much in the end process - Tokyo would still be flattened.
Of course, Mecha-Godzilla is the wild card - Miyamoto retains personal control of it, and he's rather reluctant for it to be damaged. I propose he might activate it if the situation becomes too dire, but otherwise, it'll remain inactive.
Gojira's rampages don't last terribly wrong - I'm guessing approximatey 30 Billion yen worth of damages. Enough to change the tide? Unknown - it depends on the level of PS3 failure or Wii success. (I believe we can agree XBox 360 is a nonfactor.)
That's my take on the situation, anyway.
