<i>&quot;Blockchains never shut down&quot;</i><p>Yes, they do. See this long list of failed altcoins.[1]<p>Minor cryptocurrencies can fail in some unexpected ways.[2] The classic 51% attack has happened in practice to several altcoins.[3] There&#x27;s also a &quot;burst mining&quot; attack, which can happen automatically - a big mining pool&#x27;s control system determines that it would be profitable to divert the pool to briefly mining some altcoins and get all the blocks for a while.  Then the difficulty goes way up, and the big pool goes off to do something else.  Now the difficulty is so high that no blocks get mined at all for a considerable period of time and transactions stall.<p>[1] <a href="https:&#x2F;&#x2F;bitcointalk.org&#x2F;index.php?topic=588413.0" rel="nofollow">https:&#x2F;&#x2F;bitcointalk.org&#x2F;index.php?topic=588413.0</a>
[2] <a href="http:&#x2F;&#x2F;www.ofnumbers.com&#x2F;2014&#x2F;06&#x2F;03&#x2F;thread-of-the-day-list-of-all-the-known-dead-altcoins&#x2F;" rel="nofollow">http:&#x2F;&#x2F;www.ofnumbers.com&#x2F;2014&#x2F;06&#x2F;03&#x2F;thread-of-the-day-list-o...</a>
[3] <a href="https:&#x2F;&#x2F;maxminer.files.wordpress.com&#x2F;2013&#x2F;06&#x2F;ftc_51attack.pdf" rel="nofollow">https:&#x2F;&#x2F;maxminer.files.wordpress.com&#x2F;2013&#x2F;06&#x2F;ftc_51attack.pd...</a>