What drivel. The British army has several bases in NI now and recruits from there too. Are you saying Regiments like the RIH and Irish Guards don't belong in their own country?
Their presence "on the streets" will only make things significantly worse, regardless of their intentions or regardless of whether 99.99% of them are utterly brilliant at managing crowds and interacting with the locals.
Its a fairly straightforward path to violence.
1. Hard Brexit.
2. Border posts have to be erected to manage customs.
3. Dissident republicans shoot up these posts.
4. Army deployed to guard posts.
5. Further dissident republican attacks on posts.
6. Loyalists respond by attacking nationalist areas.
7. Dissidents attack loyalist areas.
8. ***-for-tat ensues and young naive idiots (or those seeking revenge) join both sides in the droves.
On top of that, you'll have an economic depression and the border poll. The border poll will pass as the swing voters will be swayed by the economic case of joining the ROI and remaining in the EU. Violence will largely die down after that as the dissidents have no reason to exist anymore and the loyalists will know they'll never get the union back and most will see things are better off.
The best way to avoid it is to have NI remain in CU. Avoids the border posts and avoids the economic meltdown of NI.