Regardless, if someone does bid up the price of onions, it will typically trigger increased production of onions as farmers can make more profit by growing onions vs. another vegetable. This increased supply will pull the price back down.
And in the credit market - after all the good debtors are served, and there's still demand for new credit - bad debtors start being served. This keeps on going till it bursts (like in the housing bubble).
I'd just look at it in a slightly different, perhaps more optimistic way. The fact that this investment has the highest ROI means that society as a whole would benefit from injecting additional capital into that investment.
In that sense, you're right that other investors have a less desirable price. However in theory at least, everyone is better off since that investment now has more capital, and is able to produce more output, positively contributing to the overall economy and increasing the size of the overall pie.