There are thousands of methods to valuate a company, each has a different perspective. While in Europe most valuation methods focus on earnings, especially in North America cash-flow related methods prevail.
What is depreciation? Depreciation itself is an approach to to harmonize the profit/loss account and show a fair value of the asset itself. Without depreciation, investments would have a huge impact to the balance sheet of a company and veil the true fair value of its assets.
Why is depreciation no part of the cash flow calculation?
If you purchase an asset like a car or property, you have a negative cash-flow of the assets purchase price (you pay your bill). The cash is no part of your company anymore. You now have the property with a book value of X. If your property gets depreciated, does it impact your cash? Do you have to pay for it? No, it has no impact on your bank account! If your property's book value increased, does it impact your cash? Do you get any more for it? No, there is also no impact to your bank account!
"Why is it okay to ignore D&A's in the valuation?"
As mentioned in the first statement, the cash-flow valuation approach is just one approach to value a company and there are many perspectives. The DCF-approach does not claim to be completely comprehensive (which is impossible to achieve). As the rules for companies creating a DCF-analysis are exactly the same, this allows stakeholders to compare the company with other companies.
"I realize that those are not cash-effective, but they are “technical insolvency-effective"
Are they? There is no thing as insolvency in DCF. DCF is not used to measure the risk of insolvency. On the other hand (assuming that all assets get paid in the year they get purchased), the value of an asset is higher in the balance sheet than in the cash-flow statement.
Lets assume you buy a car for 14,000 and depreciate it linear over 7 years. This means that in the year of purchase your cash-flow is -14,000, while your earnings are only -2,000. The cash will never change in the following years, but the book value is reduced by 2,000. After 7 years, your total cash-flow and earnings will be even.
"if the D&A are too large over a long term, the company may consistently show negative earnings that when carried forward may exceed its equity."
See the example above, the earnings of fixed assets are always higher than the cash-flow, so the scenario just does not exist. In this certain area, DCF is a way more conservative approach than most income-statement based approaches (Which is paradox because DCF became popular because most income-statement based approaches appeared to be too conservative and to overprice risks).