Klingberg turns 32 in August. If you're not willing to get younger on your bottom pairing, where are you willing to do so?
Getting younger is always going to be a risk. That's precisely why every head coach avoids playing younger players, when they feel they have a choice. If you want to get younger, you have to be willing to take that risk. St. Ivany has at least shown that he deserves to have that risk taken at this point.
Rather than signing some veteran to displace him from the lineup, I'd rather stack up some promising defensemen that can be rotated in to the bottom pairing to see who can hack it, and who cannot. Worst-case scenario is no worse than bringing in another veteran who doesn't pan out, but without the downside of the coach insisting they be kept in the lineup due to their "veteran presence".