im talking about people who collect all collectibles (me also) and probably will collect all collectibles from all spawns and there is still quite big chance that you will be not able to finish achievement without using gems or TO. ofc you can say buy them (bells or gingerbread in TO)...
so first of all personally its not worth of it... 15k gc for 10 gingerbread? no way just for nice look of some stupid progress bar somewhere where i look once per year :)
other thing is that people who miss one or two days will not complete achievement without TO/gems (almost for sure)
and third thing if there is not enough gingerbread from all spawns for all people (even if all people catch all collectibles in event) then there will be always shortage in TO
simply it is about Pigeonhole principle... so some people will be lucky and complete their achievement, some people will buy missing items in TO and complete achievement too and the rest leads to 2 options 1st miss achievement or 2nd use gems in mayor house (what makes better availability of gingerbread in TO)... so some people have no other way for this quest except using these 2 options (use gems, miss achievement), even if they catch all spawns and all collectibles (and probably even if we all catch all spawns and all collectibles)
more exactly if you have 3 children and two breads then there is always one child without bread... using TO or not does not make diff (does not create any new bread)... only way is to give up (miss achievement)= this creates new situation -> if 1 child give up then you have 2 breads for 2 children or you can use gems what leads to the same...