The fact that Luffy suffers exhaustion after a short period of time when using a transformation is definitely an obstacle placed by Oda to restrain his power and create tension in the story. This physical limitation prevents Luffy from simply defeating his opponents easily and quickly.
This isn't the first time it has happened, and I don't know why people only started complaining now. In Wano, we literally had the same thing. Firstly, Kaido defeated Luffy in chapter 1014 or 1015 and threw him out of the island, even watching him fall into the sea. Secondly, just like Oda is making Saturn and Kizaru look like idiots here, he made Kaido magically become stupid and only remembered to decapitate Luffy and declare his victory after throwing Luffy into the sea to drown. Meanwhile, Luffy was rescued and fed by Caribou (literally eating). Kaido then went down to wipe out the entire alliance and simply "killed" Kinemon, destroyed Kinemon; Kinemon's case is similar to Kuma's, but Kaido also pursued Momonosuke and Shinobu to the extent that both jumped off the island to try to escape... The writing in Wano is better because at that point, Luffy didn't have Gear 4, and indeed, Kaido left him unable to return to the island anytime soon; literally, he depended on Momo jumping off the island and later flying back with Luffy, all while Kaido beat his second victim (his own daughter).
The point is, since the beginning of the arc, Luffy was introduced to the food machine, so he was already aware that such a thing existed on the island, which wasn't the case in Wano. After that, as some people have already explained in the topic, Luffy didn't really need to risk his life and exceed his limits and enter Gear 5 again when the option to restore his energy was literally presented to him at the beginning of the arc; this is a measure that requires less of him. Nobody would like Luffy to be that overpowered guy who solves everything and never receives an "L." Look at the development Kizaru had while Oda restrained Luffy with jokes like "ate too much"; this would never have happened if Luffy had restarted his heart because if he had done that, Kizaru would be beaten up again by Luffy.
But don't get me wrong, it's not that Luffy took an "L" for being almost unable to move just because he "ate too much." Basically, the food has already done more than Kizaru against Luffy throughout the entire arc. However, the issue of restriction imposed on Luffy by Oda is too true, and the EGG is proof of that. We have Snakeman without Advanced CoC, Base Luffy without Advanced CoC throughout the entire arc, and even G5 without Advanced CoC against Lucci and Kizaru, with Oda deciding to release some of the restriction on Luffy towards the end and near his limit.
It means I see this as the only way for Oda to develop other narratives and add tension to the arcs from now on, although it's no longer working because we're already tired of this same formula being applied in every arc. However, removing the restrictions on Luffy would make this manga even worse because basically Luffy would be going around delivering one-hit KOs to everyone left and right. Take Kizaru for example, if it weren't for Luffy's time limit and in the same context (with him trying to assassinate Vegapunk and Luffy trying to protect him), luffy would have simply defeated Kizaru in 2 attacks by now, and we would be seeing him defeating Saturn too. That would be a rather funny way to ruin the manga. Of course, Saturn and Kizaru are also strongly contributing to the arc being so rubbish because Kizaru is an empty shell that doesn't contribute anything to the progress of Luffy's dream, and the same goes for Saturn.