That whole argument is built on a flawed premise. To enter a gurudwara, one doesn't need to convert to Sikhism at all. I've entered gurudwaras as a Hindu just fine. And so, even in this sense of Sikhism as a welfare system, there is no reason to convert, as converting and abandoning one's old gods causes no major difference in being able to enter a gurudwara.
Furthermore, mandirs were beginning to adjust to this system, and were themselves beginning to have an area for eating food blessed by God (prashad), a logical extension of getting prashad from the mandir as part of standard ritual. Of course, they never held such a focus on equality, as there is no major Hindu tenet in that regard. Hindu temples today have such sections, in no small part to combat competing religions.
Furthermore, Sikhism made up a majority nowhere until after Partition, when Sikhs left Pakistani Punjab, and even Indian Punjab was still Sikh-majority until Himachal Pradesh and Haryana were broken off, and Punjab, India, is still only 56% Sikh. So, while your point on Sikhism losing adherents is valid, and it may very well grow, it will never be able to become the religion of the majority of ethnic Punjabis, much less the Pahari, Kashmiri, and Haryanvi.