The Kursk incursion took so many troops from the defense of the Donbas that the front lines there can no longer be held up. It has also diverted scarce artillery ammunition which the Donbas defenders now lack. The Ukrainian defense in Donbas is currently crumbling with the Russian side taking several villages per day.
There was some hope expressed that Russia would divert troops from its Donbas attack towards Kursk. But the Kursk operation is on Russian ground where the Russian army is allowed to use conscript unit to defend the country. Conscripts are not allowed to take part in the operation within Ukraine. This gives the Russian army a large reserve that it can use against the incursion.
The teenage conscripts in Kursk just surrender.
And your definition of "crumble" is a new one. The Russians fighting in the Donbas area are paying a huge price for tiny gains. The Ukrainians have taken more land in Russia in a few weeks than Russia has been able to take in the Donbass in months of effort.