The alternative is to water down the census questions, which also leads you down the same path (i.e. manure as data).
Check this then:
If the census is responsible for allocating federal funds and congressional apportionment, what are the incentives for making census data private and encouraging people that would otherwise hide their identity?
Now think about the data you could collect and the decisions you could make based on this data to ensure a better future for all in this country; in fact, this is a stated goal of the survey that you either didn’t know about or are willfully ignoring.
On the flip side, think about the repercussions of tainting this data and basically wasting such a valuable chance that won’t come around again for another 10 years.
(Do you reject that? As someone who uses the phrase "counting illegals" I imagine you would be interested in knowing what that number is.)
Counting illegals is not possible under the Census currently or in any point in the future most likely
https://www.law.cornell.edu/uscode/text/13/9
https://www.census.gov/topics/population/foreign-born/about/...
So unless you're willing to also say that counted illegals cannot used for either of those, then you're just being obtuse.
But if we can agree that they cannot be used for that then sure, lets identify and count them. If we can't identify (make non-private) and count them then why should we trust that those counts are accurate?
You're trading a chance of accuracy (good faith handling of data) for a guarantee of non-accuracy