Folks ponder that question and come up with their own answers.

But, in the end, if one rationally thinks through the liberal hype there can be only one answer.

First and foremost...the very term 'illegal' says it all. 'Illegal' only has one meaning.. "an action against law".

"Alien" means in effect 'foreign to a locale'.

Since words mean things, the liberals are attempting to replace the words "illegal alien" with many terms, such as 'undocumented workers', 'humanitarian relief program' and several others less descriptive of reality.

As such it is not surprising that the locales which proclaim themselves as such are hotbeds of liberals and snowflakes, ie, 'Democrats', who see but can't read the handwriting on the wall that clearly states that they are done for. Their resident working population hates them with a vengeance.

Thus, this dying breed of liberals devise the sanctuary plan to 'buy' votes to stay in political power with funds they confiscate from working locals, while the illegal aliens attack society with gangs, drugs, robbery, home invasions, rape and outright mayhem of all types. This was never made more plain than the recent Soros financed 'riots' in DC.

People with an IQ of room temperature or greater can understand
this simple fact. "If you don't work, you don't eat". Yet, the liberals proclaim that we have to feed the leaches anyway.

Cities, counties and entire states have adopted this sanctuary philosophy. And if you look, all of them are run by democrats.

So will the sanctuary idea succeed? No.

And not just because of President Trump who is controlling the purse strings of the Federal government and saying 'no more'.
Eventually even the Federal Government will run out of other people's money to spend.

The ultimate reason liberalism will fail will be because the people will demand that it fail, and will use whatever LEGAL means it can, up to and including the Second Amendment.