CustomerP are generally too cash poor to be good customers. They are going to nickel and dime you for any project that you do for them because they are either too cheap to invest in newer technology or too poor to do so.
Latest statistics indicate that Internet Explorer has less then 15-20% of market share, with versions older then IE 10 being just 2.5% of the market. Looks like IE 6 is under 1% now.
It was similar arguments that massively hampered the adoption of Linux, Netscape/Firefox,
It's one of the reasons general adoption took - and is still taking - so long.
It's also one of the reasons that companies that DID support them ended up with an edge on their competition, becoming some of the big-name companies in their markets.
Becoming market-dominant and ubiquitus includes not dropping substantial chunks of customers because you perceive them as "marginal". If you support 90+ percent of the market and your competition supports 70%, you keep getting little extra advantages. The outcome of competition is driven by tiny margins.