To me, this seems like a nobrainer. But, then I'm not a nationalistic person and generally view nationalism with the same sort of disdain as I view malaria.
NFL footballers are kneeling the flag, because racism continues to be an endemic problem in the US. Magnified, by the current President who appears to be a bigot of the highest order.
People want NFL footballers not to kneel, because they're concerned about "disrespect to the flag." As an outside observer the phrase "disrespect to the flag" comes across very much like an excuse to ignore the issues being raised. Issues which are vastly more severe than people being "disrespectful" to whatever deeply subjective notions people attached to the flag.
It seems to me simple enough that if people are sufficiently upset to kneel to the anthem/flag/whatever, then the issue is serious. And that addressing the issues causing them to kneel, will in turn, cause them to stand again.
The problems with racial inequality in US society go back centuries. In modern society, post the 1950s, NFL players have been "respecting the flag" and yet the problems of racism continue to persist.
When people start kneeling, why insist they stand? That is just a perpetuation of the status quo. If the President came out and said something like "ok, I get the message, we'll address this issue as a priority, providing you stand up" then there would be some merit to asking people to stand. Instead, Trump doubled down and threatened these players instead. Which if anything, only gives further justification to why they should be kneeling in the first place.
The notion that "people sacrificed for the flag" to give it a totemic quality that means respect for it should never be violated doesn't wash. The people who sacrificed gave their lives for a varying set of principles and values. Some people gave their lives for the flag because they had no say in the matter. A lot of poor black people gave their lives for the flag as a result of conscription and I doubt very much, whether they would be complaining about NFL football players kneeling now about rights and equality for black people. After all, the only reason so many young black men were conscripted was because they were poor and uneducated which are, in turn, the products of inequality in US society. Indeed, it seems rather crass and illogical for a black NFL footballer to stand in respect of a flag/anthem, that evidently doesn't respect all of the lives sacrificed for it.
The Americans complaining about NFL fooballers kneeling, are just playing pick'n'mix with which "sacrificed" lives have value to them and which do not. I'm willing to place a safe bet that the lives with "value" are those of loved ones and the sacrifice of "all" lives for the flag is viewed through that lens. When a white person complains about the disrespect to lives "sacrificed" for the flag, it is fair that the lives they are thinking about are "white" lives. Not out of racism but, simply because of how people's minds work. They're not thinking about sacrificed black lives and certainly not thinking in the context in which those sacrifices were made.