Easy to understand for a fluent speaker, but a learner might struggle.
We saw back when we had keypad phones, the youth would write "txt" speak because it was faster to type with 10 digits. I'm pretty sure there was a decline in literacy rate around this time, the youth struggled with spelling because they wrote rarely, but texted frequently. Smartphones fixed that problem, because they provide the full keyboard and auto-correct.
My guess is, if you took the tones out of pinyin, then a generation or two later there would be less literacy. Children would struggle to add the tones even though they know how to speak the word. Writing already contains far less information than speech. Over several more generations, the speech could even change because the written word has lost the tonal information. Compared to the past, we read far more, speak less, and write even less, and most writing has been replaced by typing.
Most importantly, you can always pick a simple, predictable sentence, or one with enough redundancy to "prove" that point. Some everyday simple sentences might work in pinyin even without the accents for tones. Try an excerpt from a patent application and I'm sure even with tones you'll fail.
> mchncl lmt xsts t th wdth f sngl xhst prt, t bt 62% f th br dmtr fr rsnbl pstn rng lf.
> Th rd vlv s smpl bt ffctv frm f chck vlv
That's just from a Wikipedia page I have open from earlier. Already quite a bit harder to decipher.
