they have a kill switch embedded inside the miners, the miner calls home every time and if you block this, the miner will not start, i know, i've tested it.
If I recall correctly a patch for this backdoor / vulnerability was released as soon as it was made public. That was more than a year ago and as far as I'm aware of that was pretty much it, with no further reports of underhanded code changes or suspicious network traffic after that.
Looking at the Antbleed website subverting this backdoor / vulnerability has been as simple as adding a loopback entry to the /etc/hosts file [1], making blocking it a fairly easy affair.
[1]
https://www.antbleed.com/GPUs are general purpose computation devices, it can programmed by anyone, it can be installed by anyone.
...and there are only
two big manufacturers of which only
one creates GPUs that are really worthwhile for mining.
ASICS are not, only bitmain can design, build and program
There were other ASIC manufacturers before Bitmain and there will be other ASIC manufacturers after Bitmain. Current market dominance does not necessarily mean that they will retain it, especially as the market grows and becomes interesting for hardware manufacturers outside of crypto, many of which are far larger than Bitmain.
Bitmain's dominance
is a problem, but that doesn't necessarily mean that the problem lies with ASIC mining itself.