user52586: SegWit2X has been called a short-sighted & severely lacking proposal. There is nearly universal preference for its alternative among those listed on the segwit support overview (SegWit, BIP 141,148,149).
Yet SegWit2X has 90% mining support.
Is this because the vast majority of Bitcoin mining / hashing power is concentrated in a few hands? Does this mean that those who control the mining power can get whatever they want?

user50934: The only answer can be "yes" and "no".
https://twitter.com/barrysilbert/status/876041414412562432 

Chinese miners representing over 80% of bitcoin network hash rate have reconfirmed their support for SegWit2x

This is an interesting one because the 14 pools they list vary from Bitmain (Antpool) with ~17% of the total network hashrate, to people like 1Hash with <1%.
https://blockchain.info/pools
Shows 4 pools (Antpool, BTC.TOP, F2Pool, BTC.com) have more than 50% of the hashrate. If you include 3 more (BTCC, Bixin, BW) you're at ~70%.  AFAICT none of these will let the users decide how they vote*, after these pools is Slush (~4.8% over last few days) which allows miners to choose what to "signal" via the coinbase transaction.
As Pieter Wuille points out, they can agree on what software to run, but they don't have the authority to change the protocols, there is nobody to "allow" or "disallow" it, they do as they like, the markets will decide if their coin is worth buying.
*F2Pool is interesting, as they did poll users to see whether they should signal SegWit, after the vote they started signalling, but this is superseded by SegWit2x.

user208: I feel that the existing answers here fail to highlight that while (the collection of) miners have the authority to select the order of transactions, and impose additional validity rules, they do not have the authority to determine what the rules are.
Segwit2X (in my understanding, there seem to be multiple ideas out there) seems to be a combination of a soft fork (activating SegWit) and a plan to do a hard fork later (changing some resource limits). The first is something that miners can do, but the second will only be accepted by full nodes that implement the same rules, and be ignored by everyone else.

52586: <p><a href="https://medium.com/@thepiratewhocantbenamed/my-thoughts-on-your-thoughts-17474d800dda" rel="nofollow noreferrer">SegWit2X has been called a short-sighted &amp; severely lacking proposal</a>. There is nearly universal preference for its alternative among those listed on the <a href="https://en.bitcoin.it/wiki/Segwit_support" rel="nofollow noreferrer">segwit support overview</a> (SegWit, BIP 141,148,149).</p>

<p>Yet SegWit2X has <a href="https://blockchain.info/charts/nya-support" rel="nofollow noreferrer">90% mining support</a>.</p>

<p>Is this because the vast majority of Bitcoin mining / hashing power is concentrated in a few hands? Does this mean that those who control the mining power can get whatever they want?</p>
 50934: <p>The only answer can be "yes" and "no".</p>

<p><a href="https://twitter.com/barrysilbert/status/876041414412562432" rel="nofollow noreferrer">https://twitter.com/barrysilbert/status/876041414412562432</a> </p>

<blockquote>
  <p>Chinese miners representing over 80% of bitcoin network hash rate have reconfirmed their support for SegWit2x</p>
</blockquote>

<p>This is an interesting one because the 14 pools they list vary from Bitmain (Antpool) with ~17% of the total network hashrate, to people like 1Hash with &lt;1%.</p>

<p><a href="https://blockchain.info/pools" rel="nofollow noreferrer">https://blockchain.info/pools</a>
Shows 4 pools (Antpool, BTC.TOP, F2Pool, BTC.com) have more than 50% of the hashrate. If you include 3 more (BTCC, Bixin, BW) you're at ~70%.  AFAICT none of these will let the users decide how they vote*, after these pools is Slush (~4.8% over last few days) which allows miners to choose what to "signal" via the coinbase transaction.</p>

<p>As Pieter Wuille points out, they can agree on what software to run, but they don't have the authority to change the protocols, there is nobody to "allow" or "disallow" it, they do as they like, the markets will decide if their coin is worth buying.</p>

<p>*F2Pool is interesting, as they did poll users to see whether they should signal SegWit, after the vote they started signalling, but this is superseded by SegWit2x.</p>
 208: <p>I feel that the existing answers here fail to highlight that while (the collection of) miners have the authority to select the order of transactions, and impose additional validity rules, they do <em>not</em> have the authority to determine what the rules are.</p>

<p>Segwit2X (in my understanding, there seem to be multiple ideas out there) seems to be a combination of a soft fork (activating SegWit) and a plan to do a hard fork later (changing some resource limits). The first is something that miners can do, but the second will only be accepted by full nodes that implement the same rules, and be ignored by everyone else.</p>
