The Doomsday Argument works because the result (Doom Soon or Doom Late) affects the number of observers, and we can work backwards to update our beliefs on which one will be.
That is why replacing the words with ponies and gold won't work unless you are a pony or sentient gold.
For my argument I just replaced "existing" with "not having gold and ponies" and drew parallel not between "specific rooms being occupied" and our civ existing at specific time but between "specific rooms not being emptied of gold and ponies" and "our civ having decent tech at specific time".
It works. Use your own substitutions.
Phisolophical reasonings often latch onto inexplicit meaning of words. If you replace them with different words that fullfill same role in language but have different meaning argument becomes much less apparently profound or even false.
You are in one of the rooms, open your eyes and see that there is no gold and ponies around. You take a look at your room number and see 7. Then you know that best decision is to operate on assumption that there is 91% chance that only 10 rooms were stripped of gold and ponies. So lots of gold and ponies should await you very soon as you move to further rooms of increasing numbers.
Reassured with that bulletproof argument we may look into the future with hope that rather sooner than later there will be times where we won't be confined to this dull rock we evolved on, to this lame rapidly decaying bodies with machines that push those clunky electrons around that can barely add 0 to 1 while overheating immensely.
Because near future is (likely) full of gold and ponies.
I hope this illustrates why purely philosophical arguments are empty of any utility. If you reason without experimental data to anchor your reasoning to something real you end up with reasoning that works just as well when applied to the thing you had in mind when it wandered as to thing opposite to your intention giving opposite conclusions and teaching us absolutely nothing about world.
Ah. First two dowvotes and no comment from fans of realworld importance of philosophy in this day and age. How classy!