J&K is and was not the only state of the Indian union beset with terrorism and other security challenges. Punjab suffered from the same crisis and didn't have to lose its statehood while successfully overcoming the challenge. We also have the example of Assam and North Eastern states.
I will readily concede that cross border terrorism by heavily trained and armed terrorists, is more than what regular state law enforcement agencies are trained to handle. Which is why Army and other central forces are at the vanguard of the counter terrorism operations, operating under the cover of the most generous constitutional immunity . I don't believe that Indian constitution is so emaciated that a threat of terrorism is enough to warrant shaking it to its core.
The Punjab state issue is not comparable to J &K because of the international element and an ideological state supporting it. Punjab insurgency never had a potential to start 3 wars with another nation and to bring union to blink of unacceptable danger and cost, but J &K have. Also, Punjab police & state government did shown competency to a level and to be fair, it's the Punjab police that finished off terrorism. I am not even gonna start compare it with Asam insurgency
What needs to be finished off, has to be finished off. And no, federal union is just fine. This country created countless states and divided it without any resemblance to the word consensus of the inhabitants of that particular state. Just few years ago, Andra Pradesh was divided with a quite a few peoples dead and billions worth properties destroyed in protest.
Back then peoples of Telugana support it, rest of Andra (who is majority) oppose it since they loose power and money
Situation here is same. Jammu & Ladakh support it, valley (who make majority) oppose it since they loose power and money
Whether or not it violated constitution will be divided by the SC. As of now, it doesn't look like it did. Goverment has asked for extensive legal advise before making this decision and if it is in constitutional, it will be struck down, just like they struck down many other laws/actions.
Safety and integrity of the union & peoples is what paramount to Union goverment,not anything else.