I don’t know that you could port Bitcoin onto Grin. So it is a threat.
But their monetary policy is fuxxed. Big mistake having an endless emissions. Same trap Monero fell into.
So that significantly reduces the threat.
Been watching the Grin Github devs' feed for months and was actually relieved to see the genesis block was today (finally), so not long to find out how it fits in.
I think the endless constant emission design was a surprise (negative one IMHO) but not at all like Monero's tail emission - which is by contrast a minimal amount, far less than lost coins would be and it's arguably possible it 'may' prove to secure the network long term as a miner incentive that other POW projects lack. Grin seems not to want to even scale down at all. We will have to see if this is fatal, but whatever the case for Grin, I personally don't think Monero's is comparable.
As for if it takes away from Monero's position as the only credible privacy coin - I don't know. If it becomes an optional side chain tech for Bitcoin, then maybe Monero is safe with a role as a stand-alone tech with a use case.
Disclosure:
I should admit I have been trying to mine Grin today, but to my great shame I am hopeless at CLI stuff and have given up after syncing the miner to the chain but not being able to work out how to make the damn thing actually mine. This is of course due to a level of incompetence in command line stuff that most here would no doubt find astounding.
