There's nothing wrong with using an older kernel, as long as the line is supported/maintained. In some case they are, most of the time they are not. And you do not want to use an older/unmaintained kernel, because it will not receive bug fixes, including security fixes.
In the particular case of the Jessie kernel, Ben Hutchings posted on this topic. In particular, he said:
Q: Will Linux 3.16 get long term support from upstream?
A: The Linux 3.16-stable branch will not be maintained as a longterm
branch at kernel.org. However, the Ubuntu kernel team will continue to
maintain that branch, following the same rules for acceptance and
review, until around April 2016. I can continue maintenance from then
until the end of regular support for 'jessie'.
The Debian Wiki Release page says the following about Jessie:
~June 6th 2018 (full) / ~June 6th 2020 (LTS)
So presumably "regular support" would be till June 6th 2018, and Jessie will also get Long Term Support (LTS) till the end of April 2020.(Note that current policy for Debian releases is for them to be supported for a year after release. Note also that LTS is not an official Debian project.) So presumably the Jessie Linux kernel would be supported till then. However, if you are running Debian 9 (Stretch), you will have to arrange to receive updates to the Jessie kernel. Since you are not tracking Jessie any longer, those updates will not be automatic.
If a newer kernel is crashing, you should first report it as a bug report. You could also spend a little time trying to debug it. This helps the Linux Kernel project and the users of the Linux Kernel. Staying with an older kernel indefinitely is of course not an option. Eventually, you'll have to upgrade to a more recent kernel, for any of a variety of reasons - including because you require more recent hardware support than is available in the kernel, because you want a feature that is only present in a more recent kernel, etc.