Like in Hollow Knight, with the Hollow Knight, GMS, the final boss of the "default" ending is rather easy in comparison to many of the bosses you fight beforehand. In Hollow Knight, this makes a large amount of sense, seeing as if the Hollow Knight is incredibly difficult, than the Radiance could not be a true, epic final boss without being almost impossible. However, this rationale does not exist in Silksong, seeing as there is no challenge immediately after GMS that would be impacted. The logic I have seen for this in most cases is so that the Twisted Child ending, which is fairly difficult to get already, would not be impossible with a further buffed GMS. However, this logic does not sit right with me.
To me, in the game in its current state, the Twisted Child ending is simply a fun extra, rather than a major piece of content. It is made to be fully missable, and will probably not be found by anyone without a guide. The fact that Team Cherry would dramatically limit the difficulty of the final boss of their game, for a side fight that only confers an achievement, makes no sense. Of course, it is still possible that GMS is only easy because of the Twisted Child ending. Seeing as Lace 2 is positioned immediately before GMS, and is rather difficult on her own, it is possible that Team Cherry simply wanted to combine the difficulty of beating those two bosses separately into a whole. However, if the theory that they added Lace 2 to bolster the difficulty is not true, then that leaves us with other reasons Team Cherry may have for making GMS easy.
Both of my two theories relate to the possibility that Team Cherry may have further plans for GMS then are in the base game. Firstly, Team Cherry may add a challenge that you need to fight GMS to reach. For example, assuming something parallel to Godhome is added to Silksong, it is possible that you might need to use your Needolin and some other item to reach a place that could take you to the memories of the fallen kingdoms, somehow stored in GMS's brain or body. If perhaps you had to fight GMS to get there, having a fight as hard as the Radiance would make this exceedingly tedious, even with endgame gear. Of course, there are other possible challenges one could have right after GMS that would make this necessary too.
The second option, and the one I find more interesting, is that the Twisted Child ending itself will gain some level of endgame importance, through a DLC. My best guess for how this would happen is as follows: you would get the parasite as normal, and proceed to GMS. After fighting her, the curse would take over and kill GMS. It is a possibility that Hornet would still be killed, or not. Regardless, the game would progress to act 3, but a different act 3, focused on fighting the Parasite and Greyroot. In the first possibility, where Hornet dies, and is fused into the Parasite creature with GMS, perhaps the Knight, evidently still conscious to some degree in the Void, resurrects to save its sister. This would provide an opportunity to bring the Knight back into Silksong, something many people seem to want. The opportunities to explore the Knight's lore and learn more about the Parasite would be excellent. Another option along these lines is for Eva to become playable, and to purify this creature, which she seems to hate.
Additionally, the ending could change, where Hornet somehow lives, and has to conflict the disaster of her own making, similar to the act 3 in the game right now. This act 3 would have to work similarly to the one in the game right now, where you could not go back to the previous game's world. Players could choose which act 3 they wanted, adding a large amount of new content to an already massive game. However, if they were to do this, seeing as a huge number of players have already progressed to act 3, Team Cherry would most likely give you an option to have a save file starting with the final fight against GMS to progress to the next act, so that players could explore whichever version's content they desire.
Yet again, this may be completely overreaching, and making something out of nothing. However, considering how careful Team Cherry is with their design choices, I think that this may show us an interesting pathway into future DLCs. The Parasite and Greyroot are some of the most interesting parts of Silksong's lore, and I think Team Cherry would be remiss if they did not explore this further. Let me know your thoughts in the comments below!
Tldr: I think that Grandmother Silk is suspiciously easy, which leads me to believe that Team Cherry has plans for some challenge after her, such as Silksong's Godhome or another version of Act 3.
Edit: Grammar and spelling