If you allowed a player to hold and open multiple maps that would have a significant impact in terms of economical value.
If say you had a party, each person able to open 3 maps each, then you could line up all the zones and significantly reduce time consumed and (while lightly) gil used to teleport (mostly a time consuming aspect).
There may be an argument in "well it's not fun that they're so much of a hassle, I'd rather they are worth less, who cares about the economical value" - but that'd be that. The hassle and inconvenience adds to the value.
Time is a function of reward rate and all that noise, that is annoying, is incorporated into the reward rate - can't remember when Yoshida said, and I know someone else already mentioned it here, but that is the reason why it is the way it is (as per Yoshida saying if they adjust the stack-ability that then they will have to nerf the rewards). Especially if you combined the ability to stack them and open many at once. Could organize yourself like 20 of one zone and knock them out one after another in much faster time. Generally as things get older I'm fine extracting time from the function, but these items were designed to be sellable and make gil - which is where I suggest more thought than "eh whatever, it's old content now- let some more people enjoy it". If you still come to that answer, then fine but it'll just impact the market value.
Which again, maybe some don't care about that.. Maybe it's the fun value over the gil value. But there would be repercussions for such a change. AS there was for crafter changes, which some people didn't like.
Most likely it'll start with a mass map buyout with a huge spike in map prices, then a influx of rare items causing their value to drop, and as people are able to slowly stack more and more maps the map value drops and the rare item value never returns. It would be great bot territory, adding to the lack of return (even if some of the items are of interest, which will cause it to pull up again over time but likely never to the value it was before). Another sketchy part of it being these chests often have gil rewards themselves- so an accelerated rate could lead to gil devaluation that hasn't been seen since 1.0, if anyone recalls 1.0 days bots did leve quests over and over that made them gil which was worse than trading for gil (generated gil impacts the economy differently). They already do some gil generation tricks, but if you had super easy chests you could stack it would certainly get worse lol.
One unique aspect of maps is they are a fairly substantial easy gil item for those who've no means yet, since they have a limit on how many you can obtain and such - also why I was suggesting ways to consume lower tier maps (to make it even easier profit for people getting new into the market, if the low tier maps can be used to do something people will buy them).
So if you do it because it's fun- and that is all, then it would be great, but if you had liked to make gil then likely no longer a great choice. Like with what happened to a lot of crafting options, which isn't a great example if someone who didn't want their crafting value to go down then also didn't want their treasure value to go down lol.. Two wrongs don't make a right. It's subjective though since some people just do chests out of fun (reward less relevant), and the whole massive inconvenience is a barrier to that.
/shrug ? lol. Personally think they should be careful with the system since it seems like a good choice to make money, but adding QoL without messing it up would be nice. Yet it's fine if they crash the market for me in the end, I don't need gil and now you are letting me buy more stuff cheap. Free discounts lol, just feel you'd be losing an opportunity to get into the larger gil market (like what happened to a lot of crafting, though certainly not all).