Merge mining works best with weaker projects that cannot stay alive and secure alone (due to lack of interest) and need another bigger project to stick to and stay alive with its help.
I'm not sure about that. Dogecoin for example also has chosen to become merged mined with Litecoin, even if it has a larger market cap than LTC (but Doge probably chose that because of its own volatility which can become a risk if it stays independent). Thus there already signs that strong altcoins develop some kind of symbiotic relationship if the move is planned correctly. This includes taking into account the "community side" of cryptocurrencies, at least both project leaders (devs, foundation if available ...) shouldn't behave in a maximalist way and support the other coin.
Afaik however BlackHatCoiner is correct about the infeasibility of a Bitcoin-Monero merged mining mechanism. However, it is probably possible to create a merged mined SHA256 coin supporting ring signatures and other Monero-style privacy features. This coin could develop into a true Bitcoin sidechain if a dynamic federation algorithm (Threshold's tBTC, for example) is added. In this case it could acquire sufficient strength to become valuable on its own and increase the income of Bitcoin miners.