You really have to understand the history of the island of Ireland to see why a hard border just isn't viable, but that's almost impossible to explain in a single reddit comment. The population is almost split evenly among ethnic Irish/British (or Catholic/Protestant) lines. Since the creation of the entity of Northern Ireland itself, both communities feel like the other should not be there, and thus sprang the paramilitary groups such as the IRA and the UVF respresenting the extremes of each community.
The Good Friday Agreement was created in order to bring peace to Northern Ireland, to end paramilitary campaigns and for both ethnic communities to live in peace. If you felt Irish, you could be just that, likewise with British. A deal the likes of has never been seen in world history. The IRA had no real reason to exist anymore, and thus neither did the unionist paramilitaries.
Now, let's throw a hard border into the mix, and spring up all kinds of complications and grey areas. All of a sudden, Irish communities on the border cannot travel freely around the island that they feel is their homeland, families, communities split up. People who travel to the Republic for work now have massive border delays, and may even have to quit their jobs in industries that just don't exist or aren't as prominent in Northern Ireland, and this particular problem isn't just exclusive to the Catholic community either. But the biggest problem of all this, is that you basically rip up the GFA and essentially force the ethnic Irish community to be British (what happens in regards passports? Will everyone in Northern Ireland be forced to get a British passport? Nobody knows), and with the DUP's decisive role in Westminster and thus Northern Ireland, you are essentially rewinding the clock in Northern Ireland to the 1970/80s.
So whilst I can see the point of view of why appease terrorists with the hard border, the situation is much more complicated than that. Northern Ireland is a hornet's nest pacified by the Good Friday Agreement, stirring it is not a good idea.