A three part question:
Which web hosting companies have been around the longest?
What are the main reasons they have survived for so long?
How much weight should be given towards credability based on their longevity?
A good amount. A host that's been around for a while is likely to have what it takes to survive. That doesn't mean its service will fit your expectations of quality though.How much weight should be given towards credability based on their longevity?