I've mashed as normal and sparged wort at approx 80C and done a few different things with it in the past. A few times I've soured with LAB from various sources (probiotics, yoghurt) without great temperature control therefore it took an age to sour and a few (1 in 3?) started to show very minor signs of fermentation by day 5. They were relatively clean.
Other times I used grain with similar results and though the grain tends to be more aggressive in souring the risk of fermentation was about the same (1 in 3) the one time it did I got 5 points of fermentation by day 5. This was much more wild tasting.
Other times I've used co2 purging, high temperatures (40-44C) and bacteria from similar sources including grain. These were much cleaner and quicker and didn't show signs of fermentation because there is no day 5 business, souring is done by day 3. Grain souring can be relatively clean if you keep the temperatures in the range that encourage and promote LAB.
In all cases these worts were boiled and hopped before a regular fermentation. What is happening is the 80C sparge is enough to somewhat pasteurise the wort. I believe the phrase is to not eliminate, but reduce the presence of unwanted microorganisms to a statistically insignificant amount. By adding back the ones I want and controlling the environment I get the outcome I want before what survived the sparge produces too many off flavours. In the case of the grain souring I'm relying on the environment and the desirable organisms thus outcompeting the undesirable ones, but in all cases without boiling it as soon as I'm happy with it the end result would be an unpalatable progressing spoilage.
Back to the OP. If you mash in and don't reach a high enough temperature to kill off a significant enough quantity of the wild yeast and bacteria present on the grain you'll find that it starts to sour and eventually ferment. If you can keep it hot and prevent oxygen ingress you'll favour LAB and it'll sour, dissuade enteric bacteria and yeast and you are basically sour mashing which is a thing. If it gets too cold then you'll favour enteric bacteria, yeast, it might pull a vacuum as it chills allowing further contaminants to access it and it'll get gross. Mould is very common in the presence of oxygen.
The chances of mashing, allowing the mash to cool to fermentation temperature and letting it ferment out and getting something delicious is really really low. You've also the issue of the yeast doing the work having to grow up in the mash extending the window for which things get weird and most likely mouldy.
If you have these wonderful ideas it is sometimes worth figuring out what goals you are trying to hit here. If it is about making an otherwise clean brew, but fermenting using the yeast present on the grain it is better to culture up some grain, streak it out on plates, select yeast colonies, check you've achieved a monoculture, propagate up this yeast, perform fermentation trials, proceed to a full size batch. That way you've reached your goal, but not by rolling dice again and again to get there. If it was to make something a bit weird and funky you'd probably be better performing a sour mash, sparging and boiling and fermenting as usual with a known yeast. You'd have the flavour impact of the sour mash, but a reliable outcome. If you wanted to grain sour, you could just grain sour, the choice to boil afterwards is yours, but I certainly would if my bacteria source was grain.
I don't always boil my sours any more, especially if the bacteria source was clean. I am happy to consider a hot sparge as pasteurisation and sour under controlled conditions. Once enough yeast is in there and working everything else is outcompeted and the alcohol, low pH, co2 and maybe a dry hop do the rest. It is a bit more risky, they always taste a little more cheesy than my super clean boiled sours, but still beer.
For my clean sours I mash, sparge, boil, sour, boil, ferment. First boil is based on DMS potential of the grist and if I intend to use copper finings so 5 - 30 minutes. Second boil depends on hopping and evaporation goals so can be as long as an hour, but usually is just 15 minutes to sterilise.