Danny Sabbah, a 30-year veteran with the company—his most recent title was chief technology officer and general manager for IBM’s cloud unit—also retired in late December as did Brendan Hannigan, general manager of IBM’s security group, according to their LinkedIn profiles.

Sabbah did many things at the Armonk-N.Y. based IBM since joining it in 1988. At various points he ran its next-gen computing initiative, Tivoli systems management software, and the Rational development tools group, for example.

Hannigan joined IBM four years ago when Big Blue acquired Q1 Labs, a security firm, he had led as president and chief executive.

Fortune reached out to Sabbah, Hannigan and IBM for comment and will update this report if any of them respond.

IBM (IBM) is a big company, and it’s no surprise that anyone with 30 or more years of tenure would be ready to retire—as Mills and Sabbah did. But some insiders worry that with these departures the company is losing critical experience that will not be easily replaceable.

WATCH: IBM CEO on the most disruptive technology:

According to one former IBMer who is still close to the company, the big story is that lots of top execs are leaving but it’s unclear whether the company can attract new talent.

Fortune did learn from IBM sources that Jim Comfort, another company veteran, will step in as CTO of cloud (Sabbah’s spot) while Marc Van Zadelhoff is now general manager of IBM Security, filling Hannigan’s shoes.

So IBM still has a bench of company long-timers but skeptics are unconvinced about its ability to forge ahead in a world where new skillsets are required.

In a research note written after Mills’ retirement became public, UBS analyst Steven Milunovich summed up IBM’s quandary. While crediting Mills with boosting IBM’s software business both organically and through acquisition, Milunovich also pointed out that the group’s performance had underwhelmed of late.

UBS expects that IBM Software’s pre-tax income will decline nearly $2 billion over the past two years, sinking from about $11.1 billion in 2013 to a projected $9.2 billion for 2015. (IBM will disclose its fourth quarter and FY 2015 earnings earnings on January 19.)

“IBM is getting picked off by upstarts like ServiceNow and investors wonder if there is a need for middleware like WebSphere in a cloud world,” he noted. Middleware is tech jargon for a category of software that works as glue to connect other, disparate pieces of software so that they can work together.

Perhaps more important, Milunovish noted that, for IBM, bringing in “new blood may be helpful.”

But that’s more easily said than done. It’s hard for IBM to attract the best-and-brightest engineers and programmers who are being wooed by companies like Google (GOOG), Facebook (FB), as well as Silicon Valley startups that can offer stock options and other perks attractive to young developers just entering their careers.

No doubt, IBM would say it’s had its fair share of talent “gets” of late. A few months ago, for example, Fortune reported that John Considine, a respected cloud executive who founded CloudSwitch and sold it to Verizon (VZ), came aboard as vice president of IBM’s Cloud Innovation Lab, for example.

Still attracting lots of new talent while also paring headcount as it’s been doing, is a huge challenge for IBM chairman and CEO Ginni Rometty, who took the reins in 2012. On her watch she has had to refocus the tech giant on the same cloud and big data analytics opportunities that all of its competitors are also targeting while managing declining businesses in other areas.

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IBM’s dilemma in trying to break into new arenas while keeping older businesses going, is not unique. Long-time competitors including Hewlett Packard (now broken into two entities Hewlett Packard Enterprise (HPE) and HP Inc. (HPQ)) and EMC (EMC) (navigating a $67 billion merger with Dell) are all navigating the same turbulent waters.

MORE: On IBM Software.

Microsoft (MSFT), another long-time rival, is doing better than the other traditional IT companies in this scary new world, probably because it didn’t have a huge legacy hardware business to support and has been able to milk cash-cow franchises like Windows and Office long enough to fund its Windows Azure cloud and other modern efforts.

But all of the aforementioned companies now face a daunting challenge from Amazon (AMZN) Web Services, which has built a huge cloud computing business that hit $2.9 billion in sales for its most recently reported quarter.