there is some prima facie logic to this conclusion, but it's wrong. ever since 2011, dozens and then hundreds and then thousands of altcoins were launched. with each market cycle, exponentially more altcoins are created. and with each market cycle, bitcoin's price exponentially increases.
most people don't stop to consider the actual dynamics of bitcoin/altcoin money flow. if altcoins didn't exist, a lot of the BTC circulating in the altcoin market would be dumped back into BTC/USD as supply, driving prices downwards. altcoins pump the whole market.
there are other obvious examples of this "supply withholding / price pumping" mechanism. consider proof-of-stake coins or masternode coins. their markets have greatly benefitted from this dynamic.
I enjoyed reading your first paragraph because indeed the reality contradicts the conclusions of that research. But why do you think that those BTC would be necessarily dumped for USD?
because it's BTC being used for speculation/trading. without altcoin or security/token markets, the only thing left to speculate on would be BTC/fiat prices. i definitely don't think all BTC trading on the altcoin markets would return to the BTC/fiat ask side, but it would obviously be a non-zero amount. it would be naive to assume every altcoin speculator would otherwise be a BTC hodler, amirite? so based on that logic, altcoins should have a net positive effect on BTC prices.
in addition to the above, demand for altcoins can also create demand for BTC. for everything besides the top coins (which have liquid fiat markets), people need to buy BTC before they can buy into altcoins. the flip side of this = altcoiners create additional BTC supply during bear markets as they sell everything off for fiat. but really, as long any value is held in altcoins at all, the net effect on bitcoin's price should be positive.