The Windows phones still clinging to life are not Microsoft's.
Does "clinging to life" mean "existing" here?
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There is more to it than "existing".
In the medical sense, this expression is used when someone is gravely ill or seriously injured. It often suggests the person (or animal) is barely alive:
A man is clinging to life after a motorcyclist slammed into him on Sunday.
In a more figurative sense, the expression can be used when we don't expect something to be around or usable much longer. So the expression in your sentence doesn't only mean that the phones exist; the language suggests the phones are an antiquated technology and may not be serviceable for very long.