There's also the adoption perspective. If there was a cryptocurrency with the level of adoption/network effects like Dogecoin and its mature market, that would probably be a very good merge mine coin.
Yeah, I suppose that is one of the reasons why Doge is being merge mined by LTC miners (they both use the same scrypt algorithm for their PoW). But I think for miners the main incentive is still profitability, I doubt majority of miners care about popularity of an altcoin. Although the two are not mutually exclusive, a popular coin can be more profitable as well.
But higher level of popularity = more network effects, more network effects = higher possibility of retaining long term value, LIKE Bitcoin.
I believe, if it was possible, a merge mine with Monero would be the best possible choice for Bitcoin. BUT that would merely complicate things. When the actual problem comes, and if the decision would mean Life or Death for Bitcoin, whether to increase the supply limit or not, then I'm VERY confident that we'll get community/network consensus to choose Life.
