Firstly, I don't like the aesthetic. That's personal preference though, so I won't dwell on it.
The Guard is thematically defined by not getting access to the sci-fi/fantasy nonsense the rest of the setting gets, with the exception of basic energy weapons. This means that it can't really compete when it comes to interesting mechanics: once another faction enters a design space that overlaps with Guard, it produces a much more interesting and refined take on that gimmick (both AdMech and Tyranids do a much better job at having "rigid command structure" as a faction identity, for example).
One of the main points I see in the Guard's favour is that it's thematically an underdog faction, and to be honest I don't understand how that belief occurred. The Guard is in top 4 most numerous factions, and top 3 when it comes to replacability. Sure, any individual guardsman is probably dead in seconds, but that's also true of any individual Ork or gant. It feels like a Guard player can get tabled and declare that they held off long enough for some of their endless reinforcements and declare themselves the winner to an excessive degree, and that's a pretty high bar coming from a Tzeentch player. You could argue that this niche has been carved up just like most of Guard's mechanics: Genestealer Cults now lets you run a group of mostly-normal humans that get results through cunning instead of endless supply lines.
There is one aspect that Guard still has on its own, and bluntly I wish it didn't. Guard is partially the tank-focused faction, and it feels like that causes more problems for other factions than it solves for one. Guard tanks aren't exactly great, since they're designed to get strength from high numbers. However, because Guard is "the tank faction," it wouldn't do to have other generalist factions get better tanks than it. Only a very small number of factions have usable tanks right now, mostly because they can count one style of vehicle within their specialization (example: the Tau have a single good tank loadout because they surmounted guard as the "shooting and conventional tactics" faction several years ago). I want this to finally be replaced so that other factions benchmark is good tanks, not mediocre tanks that it doesn't make sense for more elite armies to field that many of.
The setting no longer uses unaugmented humans as the baseline, if it ever did. It no longer needs Guard to be the command army, the shooting army, the horde army, or the underdog army. 40K has outgrown the Guard, and it's time to stop being limited by it.
