TL;DR: job creep is nothing new, and you have to protect yourself.
This is from long ago (1990s) in a Silicon Valley far, far away.
I was a VAX/VMS system manager. hired by a chip equipment company to maintain the network of microVAXen for the chip machines' software development. This was the microVAXen servers, the desktop VAXen, and the LAN network. The development groups were in three different buildings, so I did a lot of driving.
The company was launching a new platform for their products. The goal was to use a workstation as the operator interface, so there would be networking to the various chambers, an X11 visual interface, data collection, etc. Despite all the managers in the world, Windows 95 was NOT chosen to run real-time operations, instead they went with Solaris.
I didn't know Solaris/*NIX from zip, but I was told that I would be responsible for those workstations. They hired a contractor who spent six months teaching me how Solaris worked. He and I spec'd out the workstations (SPARCstation 20s), with the peripherals we'd need to make it work (“No, we're not going to load the OS on fifty 1.44 MB floppies”). Since the company sold hardware, the software to run the workstation was provided 'free', which meant they didn't like to budget for IT infrastructure/support.
More new products came on line, using that machine interface with the Solaris platform. After several years, I was maintaining the VAX network across eight buildings (and this was still coax, which meant a lot of crawling around to find out who had unplugged the cable in their office), the growing Solaris development network, the production lines that were assembling the chip machines, spec'ing new workstations to replace the SS20s (and the managers still wanted Windows95 so that their data would come back in nice Excel spreadsheets) and getting calls at 2 AM from overseas customers who need help. There were a few other sysadmins, and I was running an ad-hoc round table every week so we all knew what's going on. On-call 24/7.
One day, my manager said that the department needed to migrate the office staff from W95 to Windows NT, and would I be interested in learning NT? I said sure, because you don't say No. So off I went to a week of NT Management training.
When I got back, and shovelled up everything that had hit the fan in the meantime, I found that they were expecting me to take over Windows support for the department. Not just the W95 to NT upgrade, but supporting all the apps, the networking, license management, installations, user support, hardware repairs. There was an IT department providing exactly this support to the entire company, but the manager for the software development department didn't want to pay the funny-money that was budgeted for that department, it would be cheaper for him to have the *NIX guy do all the office support, too.
I was taking a badly-needed lunchtime walk with a buddy. A few months back, he had been looking at another job, but was on the fence about leaving all of us stranded without his support. I told him that if he wrecked his career just because it would inconvenience his co-workers, he was an idiot, and he should check out the new job possibility (he didn't get the job, alas). So, I told him what was going to happen to me, and that I was going to go crazy, but I didn't want to abandon the people I had been working with for eight years. He told me that if I wrecked my career just because it would inconvenience my co-workers, I was an idiot.
Within a month, I had contacted the guy who had taught me Solaris, and was working for him and a team of other sysadmins at a software company. Since they sold software, they were smart enough to not consider IT support as a cost burden, and budgeted for it to grow. Plus, I rode mass transit for 20 minutes instead of driving an hour each way.
As far as I can see, things haven't changed. They'll pile the work on your back until you break or bolt. Save yourself.