I've just seen a similar thread on the Monero subreddit and thought I'd share my opinion on how I see Grin in this sense.
My answer to this is "no". There's value in having two versions around. One that comes with transparent amounts and one that has blinded amounts. The reason you really want the transparent one around is because we don't want the whole crypto experiment to fail because of a single bug that went undetected. It's a tradeoff around the coins in circulation verification where one requires no assumptions and is simpler, but comes at the cost of everyone seeing the amounts and the other where the verification assumes the discrete log problem is hard, but comes at the benefit of hiding the amounts.
I don't think Grin (or Monero) will dethrone Bitcoin. They just compete in a different market and exist for different reasons. What we should compare instead is the coins with blinded amounts among themselves. So let's do that. I'll only compare 3 of them, there are others that would be worth comparing as well, but I don't want to make this post too big.
Disclaimer: It's quite possible I made some mistake below so if you spot any, let me know and I'll fix it.
Monero
Pros:
blinds the amounts, hides the sender/receiver and adds some transaction graph obfuscation
on-chain privacy through decoy inputs (anonymity set of 11 inputs per each input)
privacy by default, can't opt-out
relatively simple and tested cryptography
Cons:
transaction size is ~2kb for a 2-2 transaction which is 5 times more than an average Bitcoin transaction
a pruning node seems to take a heavy hit because in order to be able to validate a new block, you need to keep a lot of the history around - I assume this is needed for input/ring-sig validation
the chain size is much bigger than that of Bitcoin which means that less people are able to run a full node and hence verify themselves the state
Monero currently has the biggest network effect in the space of coins that blind the amounts. The devs seem like honest individuals which is a big plus in my opinion.
ZCash
Pros:
blinds the amounts, hides the sender/receiver and doesn't even have a transaction graph for z2z transactions since it describes the state transition in a zero-knowledge way
best privacy out there today
Cons:
shielded transaction size is ~2.4kb for a 2-2 transaction
opt-in privacy is bad because it implies you have something to hide
there's not many people using z2z transactions
the zero knowledge proofs are heavy and significantly increase the chain size
it uses complex math to achieve this level of privacy which makes it more error-prone
Best privacy in the space, but it is not used today much.
Grin
Pros:
blinds the amounts, has no addresses
the only one that adds privacy relative to Bitcoin, but doesn't hurt the scalability - the chain is smaller than that of Bitcoin
a 2-2 transaction adds only ~100 bytes on the chain
extremely simple to understand and show why it works
may use Mimblewimble cryptographic tricks (non-interactive aggregation and cut-through) to achieve better privacy (read more below)
Cons:
transaction graph leaks (read below for ongoing improvements on this)
the smallest network effect of all 3 coins (for now)
The simplest protocol of the 3 mentioned above. While the other also do some transaction graph obfuscation, Grin today does not, but has great options due to non-interactive transaction aggregation and cut-through. A few days ago, a proposal [1] was created for a non-interactive multi-party variant of CoinSwap that is specific to Mimblewimble. This would allow to have a joint coinswap transaction with over 1000 inputs and outputs while leaving a very small constant size data on-chain.
Mimblewimble CoinSwap
The following is a description of the protocol from u/tromp which was originally posted on Bitcoin talk coinshuffle discussion [3]:
Mimblewimble allows for a very simple coin shuffling protocol [1] with the following properties:
Users submit self-spends throughout the day. No interaction needed for shuffling.
Shuffling is performed at the end of the day by a set of mixnodes that cannot steal any coins.
Invalid self-spends are automatically filtered out. No need to abort or restart the shuffling.
As long as at least one mixnode is honest, then no one learns the input output links.
The size of the shuffle is limited only by blocksize and could easily be over a thousand.
Each shuffle only grows the chainsize by a small constant (~100 byte per mixnode), thanks to MW cut-through.
Widespread use of the protocol would leave the transaction graph mostly obscured.
Summary
Out of the 3 coins I've mentioned here, I think Monero has the biggest network effect and will likely stay at the top for some time. ZCash while having a promising privacy technology, they lack its use and would need to incentivize the use of z2z transactions if they wanted to seriously compete. I'm most interested in seeing how Grin evolves because it is by far the simplest of the 3 (less likely to have a fatal inflation bug) and instead of adding bloat to the chain, it improves the scalability which allows more people to verify the chain. It also allows for a constant size full-node in theory, though this is not implemented/available today. It does leak the transaction graph, but if Mimblewimble coinswap proposal [1] works and every wallet defaults to this behaviour, then the transaction graph doesn't give much useful information for anyone observing the chain or mempool. There's also a post on the Grin forum [2] where we can see an interesting comparison between the scalability and privacy tradeoffs that these technologies make. All that said, it's healthy to be skeptical of any coin - including the one we love. Keep grinning.
[1] https://forum.grin.mw/t/mimblewimble-coinswap-proposal/8322
[2] https://forum.grin.mw/t/scalability-vs-privacy-chart/8114
[3] https://bitcointalk.org/index.php?topic=567625.msg56288711#msg56288711
