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 not sure what careless cleaning is ,we bought our TT just before christmas a couple of years ago.it had been erected for a year under a tree
one side was bright green no not the dye but green slime the other had faded.
for £200 i wasnt going to be picky and even if the canvas had rotted you wouldn't get a trailer for that so i bought it
we took it down in the rain and my mrs point blank refused to let it indoors until it was clean
i (yup me) spent an hour trying to clean it with grangers canvas cleaner as the canvas froze immediately (my hands were blue)
well time for tough tactics so i bought a very hard scrubbing brush and made up a gallon on hot water and two fairy washing tabs and got to work
two days later and many buckets later(the water froze almost as fast as you put it on ) and a good rinsing with boiling water (the hose froze too) it was deemed clean enough to be allowed in.
canvas is resilient stuff ,i broke all the sensible rules but the tent survived ,the greens gone as is the bird crap but the canvas is still faded on one side.
5 gallons of thomsons later though we have a good TT fit for a few more years yet.
canvas though can get marks that never come out (like natural fabrics do) rust stays and mould stains do ,they can be lightened but they stay ,the best bleach is the sun our roof clean but stained has now lightened right up now its had the sun.
we have noticed if you waterproof when the canvas isnt bone dry (dry to the touch)it lets it breath.
but if you waterproof very dry canvas (sun baked) it swells as canvas does but i think the silicone gets far further in the thread and stops the canvas shrinking all the way back as it would if it got wet in the rain so the fibres are thicker stopping it breathing so easily.

Canvas relies on tiny holes to allow it to breathe.  Soap & detergent work by stopping surface tension in water.  This is what stops water seeping through these small holes.  This is why canvas should only be cleaned with clean water, & why most camp sites don't like kids playing with bubbles.  Detergent will make canvas leak.  You will need to re proof.  This however blocks up the pores in the fabric & means it no longer breathes, making the tent suffer more from condensation as well as more difficult to dry.  A cotton tent should last many years befor re proofing becomes needed due to damage to the canvas.  In my old Scout troup, some of the canvas was over 30 years old & was still waterproof.  You may as well have bought a £50 tent from Halfords.

Steve.