But the point is in at the Home Office has a general "Hostile Policy" towards everyone : guilty until proven innocent. 


That was why Windrush happened, as people who were legitimately in the UK, but through no fault of their own couldn't prove it, so were marginalised/lost their jobs/deported/not allowed re-entry into the UK 
These were not people who were here illegally 
That's the same sort of "hostile environment" that faces EU citizens, some of whose who will have been here for decades and now need to "justify" why they are here.
And to make matters worse, even if they had previously been awarded ILR, that automatically expires post 31 June
Puts a whole new meaning on the word "indefinite" ....
And given the avowed "hostile" environment policy, that person, say for example an elderly person with ILR but with dementia (thinking worst cases), who thereforehasn't thought to jump through the hoops to get the "new" on line only right to remain, would thereafter be here illegally.
According to LouiseCollins28's naive trust in the Home Office, as well as her declared belief that the hostile environment is perfectly ok for people who are here illegally, it would be perfectly ok for the Home Office to go after such a person, as they are , after all, now here illegally as they haven't jumped through the new hoops despite having been here legally before 
There are lots of other examples of people who were here legitimately, who will now find that they are "illegal": children of EU citizens taken into care for example, if the council hasn't sorted out their status for them. Should they then be sent "home" to a country they've never known? Despite the fact that the UK is the only home they've ever known?
I could have been caught up in the Windrush scandal but thankfully, being a middle class family who enjoyed travelling, despite having arrived on a legitimate Commonwealth country passport, we naturalised as soon as we could and got British passports, so that we could travel (especially as a South African passport wasn't very welcome in many countries). If we hadn't, then we might/would have had problems at a later date.