No, this is not a good argument.
Although other conditions can inhibit decay (such as the anoxic conditions found at the bottom of some lakes), and some fossils aremore resistant to decay than others, the biggest problem is that it assumes that there is a single rate for deposition.
In fact some beds can be deposited very rapidly, but others are deposited very slowly - and geologists can tell which. Moreover we need to distinguish between the AVERAGE rate and the PEAK rate. If, a rare event drops a relatively large amount of sediment that represents a high PEAK rate of deposition - but the average rate would still be much lower. The average rate is needed to work out the age - but the average rate does not need to be high enough to produce fossils - so long as that rate is sometimes reached.