You don't need to. If you like the price of an upgrade to 20 it's likely cheaper than the 21 upgrade will be until next summer. So if you want a new version now and don't have much money consider it. I uninstalled 17 and find 19 and 20 faster and more stable.
You question is, "do I need to upgrade to 20.0?" After downloading the trial version, only you can answer that question. Does VP20 bring something new in functionality that you might be interested in?
If VP17 meets all your needs and is stable, you can stay with it if money is an issue. Then wait until finances are better, download the latest VP trial and then see if there is an improvement in editing.
I've personally found the latest version of VP20 extremely stable. And to me, that's worth the cost of upgrading to relieve the frustrations of freezing, white screens of death, etc..
Be aware the audio in version 20 is absolute a great disaster; you can not work (on a professional basis) with audio in the current build (401 and 411) in version 20.
All VST-plugins are destroyed, especially when used in combination with automation enveloppes.
Some bugs (audio related) were squashed, but this also introduced other bugs that really destroy your audio workflow.
So if audio is important, you use VST (2 or 3) and automation... consider version 17.
Versions 11 ~ 19 are very stable, audio wise (besides some exotic RX noise reduction plugins with dynamic envelopes), but 20 (since 401) is simple not doable.
Until 370 it worked okay (it did crash RX and Melda, but that's default in Vegas), but after that it got even worse (less crashes, more hick-ups and freezes).
So I audio is important, you use VST (2 or 3) and automation... consider version 17.
Huh? VST 3 doesn't work in Vegas Pro 17.
And rather than going on with your usual bagging of Vegas Pro re audio, how about just suggest that the OP install the trial version and let him/her see if VP20 meets his/her audio requirements. Sheesh!
Be aware the audio in version 20 is absolute a great disaster; you can not work (on a professional basis) with audio in the current build (401 and 411) in version 20.
I do not see version 20 as a great disaster, since we are talking here about an Video editor. If one has special requirements, then it it is hard to take audio limitations and talke about a "great disaster".
There's no compulsion to remove old versions of VEGAS. I mostly switched over to VP 20 though with earlier builds it had problems with iZotope RX VST 2 plugins so I stayed in 19 to work on audio. Those problems seem mostly addressed though I haven't had time to test it much.
You could do the same in 17. Overall speeds and stability is better for 20 than 17 and I wouldn't give up features like the color grading panel, video/computer levels being doing automatically, zooming in on the preview window, support for modern phone video formats, etc.