Keep in mind that the IRS document posted above is the tax guidance in its entirety. There is a ubiquitous assumption that in the US, altcoin for altcoin trading is taxable, but per the relevant IRS guidance:
Q-6: Does a taxpayer have gain or loss upon an exchange of virtual currency for other property? A-6: Yes. If the fair market value of property received in exchange for virtual currency exceeds the taxpayer’s adjusted basis of the virtual currency, the taxpayer has taxable gain. The taxpayer has a loss if the fair market value of the property received is less than the adjusted basis of the virtual currency. See Publication 544, Sales and Other Dispositions of Assets, for information about the tax treatment of sales and exchanges, such as whether a loss is deductible.
Notice it clearly only mentions virtual currency being exchanged for "other" property, and "property" in exchange for "virtual currency", never "virtual currency for virtual currency". It is not clear that the IRS intended this to be taxed, and it would lead to situations that have no precedent. Eg, if one buys $1000 worth of altcoin, and then does more altcoin trading for thousands of dollars out of this initial investment, but the altcoins all go to zero (which is quite common), then is there supposed to be a taxable gain? It would not make sense. Out of common sense, it should be a $1000 loss. This only happens in crypto because the volatility is many times higher than all other assets (and the volatility is high because it is purely digital and so easy to exchange). If one believes that these transactions are all taxable, then one must sell holdings before the end of the calendar year to fiat to have the funds to pay taxes, increasing administrative burden, possibly forcing one to sell crypto for a depreciating currency (fiat) and so having a loss, not to mention transaction fees.
I doubt that the IRS has considered any of this or would want for these things to happen. The situation is not the same for stocks, which are tangible companies with employees, assets, real estate, bonds, hopefully profits, and so forth, not digital representations which can be invented en masse in a day. It is clear from the above document that the IRS is not proficient in this space because of the distinguishing of "convertible" digital currency, their definition being one that "that has an equivalent value in real currency", from non-convertible digital currency, but all digital currency has an equivalent value in real currency, just as airline miles and other digital things have a value in real currency. Also, as has been pointed out, the tax on mining bitcoin at the time of mining does not make sense. In manufacturing, when one produces 1000 copies of software, there is no tax until it is sold, it is inventory until that point. So one could again be taxed on mining something that becomes worthless later. When gold is mined, it is generally taxed when it is sold, not at the time of mining.
Like-kind is a total tangent; anyone who has looked at the forms required can tell that it obviously only applies to real estate. So the recent ruling that confirms this obvious fact has no bearing on anything. For example there are tax rules on currencies. If the government comes out and clarifies that a rule only applies to currencies, it would have no bearing on crypto.
The problem is that because the IRS has decreed that virtual currency is to be taxed as property, and property is taxed on form 8949, it would not make sense to not mention altcoin-altcoin trades, because if only reporting fiat trades, the numbers will not add up because the form (which was not designed for crypto) requests the number of items, the date bought, sold, and the cost basis, unless the number of shares of the asset is not reported. The IRS likely did not consider any of this, nor how to deal with hard forks, and so on, so it is normal that the forms would not make sense.
The most obvious solution would be for the IRS to come out with a new form specifically to report crypto trading, which would deal with all of the above issues.