How to determine if my 2015 MacBook Pro can still handle current workloads?
I would look at three things

Time is money
List specific measurable benchmarks ( e.g. launch 4 specific apps each day and time them till usable, script a clean build of a representative app )
List specific app and OS requirements ( e.g. need N-2 and N+1 beta compatibility for Xcode SDK)

In the end, you’re appealing to your manager for funding and reasonableness of the “requirements” and not IT for a “blessing” of the age or benchmarks.
Consider that people are expensive to recruit and train and hardware is cheap. But it’s easy to measure hardware expenses, so that tends to get scrutinized over process (erase and install) or training.
Do some quick math on “Is it worth it?” and good luck!

In your case we would send you an identical Mac to your 2017 one that is erased and have you run your chosen tests. Most of our managed Macs need an erase every 2 years and developers tend to need erases much more often since they alter the system so heavily. They understandably complain bitterly when asked to clean (just like system admin do), which is why we send them a second kit to run tests so they aren’t forced to migrate but can do a good test on a clean system. It’s rarely the hardware slowing them down in my experience, but when it is it’s nice to have good data on a clean build so that the budget person doesn’t think the “spec” was made up or not backed by data and testing.