"it is plausible to suppose" is a grammatically correct sentence and can be used in written English.
It is typically used when you are making an assumption about something based on evidence. For example, "Given the evidence we have seen, it is plausible to suppose that the Earth is round."
In addition to the familiar reasons for fulfilling its duties of support, honesty, reciprocity etc., it is plausible to suppose that doing so also expresses and is known to express loyalty to one's friends, and that that gives additional support to the duties.
The authors say it is plausible to suppose that by 2000 developing countries were gaining around $350 billion a year in additional GDP thanks to access to the global market in goods and services.
As with much economic uncertainty, it is plausible to suppose that this could weaken the pound, reduce domestic and foreign direct investment, and increase borrowing costs," the report says.
It is plausible to suppose that convergence does not require total unanimity.
Beyond these figures with attested connections to Pyrrho, it is plausible to suppose a certain influence on Pyrrho from Democritus.
It is plausible to suppose that a subject's match to the relevant patterns will generally be a matter of degree.
It's plausible to suppose that much if not all of the value that we place on believing rationally depends on a connection between believing rationally and believing what is true (although the precise nature of this connection is no doubt a particularly vexing topic).
If one accepts a representational view of belief, it's plausible to suppose that the relevant representations are structured in some way that the belief that P & Q, for instance, shares something structurally in common with the belief that P. To say this is not merely to say that the belief that P & Q has the following property: It cannot be true unless the belief that P is true.
It's plausible to suppose that people have some sort of privileged access to at least some of their own mental states or processes: You know about your own mind, or at least some aspects of it, in a different way and better than you know about other people's minds, and maybe also in a different way and better than you know about the outside world.
Because these cases are only found sporadically in older animals (≥8 years) coming to slaughter with few or no signs of disease, it would be plausible to suppose that atypical forms of BSE may have a lower virulence than cBSE and be innocuous to humans.
