No deal on 29th March does not mean no deal is ever reached.
Granted 'a' deal will eventually be reached. That much is true.
But the issue is what sort of deal. The UK is on the back-foot. To argue otherwise is nonsensical. The EU doesn't necessarily only have its sights set on Europe-wide expansion. Its main international export is actually it's regulatory regime. There are are, if you like, two main global super powers and a third increasingly on the rise; the EU, the US and China, respectively. In order to deal within the Single Market others must comply with EU regulation. Ergo, whilst the UK sets sail in the world to forge all these wonderful trade deals, which incidentally it won't be able to implement until it has reached a final agreement with the EU, it will still to a very large extent have to comply with EU standards.
The real kicker is that the UK will have absolutely zero input or influence over those regulations. The sadder part is that the UK actually played an instrumental role in developing those regulations.
Even if you ignore all the politics, rules and regulations aspects and look at it purely in real-world terms: the UK is a tiny nation compared to a 'group' of 27 other nations all marching to a similar beat. 27 other nations which happen to be our neighbours toboot. It therefore follows that a small nation does not have the wherewithal or even hold the cards to boss the larger group about.
People seem to think that forging 'World trade deals' will be our saving grace. It really won't. World trade deals are not a 'thing' as such; operating on WTO standards is effectively what happens in the absence of an actual trade deal. Bi-lateral agreements take years to hash out and negotiate. That's why the EU has presented the UK with essentially a menu of its current 'ready-made' trade/relationship options that it has with nations outside the EU. The problem for Theresa May, and the UK at large, is that a lot of these 'off-the-shelf' models either contravene the UK's redlines (see NI) or indeed the EU's redlines (see EU's firmly welded key pillars).
For 40-odd years the EU has put up with UK exceptionalism because despite our arrogance we actually did bring something to the table. Perhaps some lessons are best learned the hard way.