I do not think having too many quality SLAs is a burden, it is our strength. We are a formidable ODI bowling unit because of our SLA strength. If we have another capable SLA in Test (0.75 eq of Shakib), it would be very difficult for any team to pile up runs against us at home.
A good spinner is a good spinner regardless of his style of bowling. A quality SLA is definitely better than an ordinary leggie or offie. I do not think our SLA strength is preventing anyone to become a quality leggie or offie. A leggie or offie who bowls right-handed is very unlikely to change bowling arm to become SLA.