Yeah, LTI was only offered for a period of time to early backers, and these days only happens for ships if you buy them at their 'concept' phase, which means CIG has just nailed down the details of a ship, and letting people buy it at its cheapest price, with LTI. Typically the concept are 100% new, occasionally its a variant of an existing or near-complete ship (some being available to fly straight away, in the case of the Avenger variants).
Its a nice perk, but i think most people recognise it as being about 1/10th or so of the fee's, they'll all be minor, i'd doubt that ALL fees combined would be more than about 20% of what you'd earn doing random activities. It most likely wont be possible to fly without insurance (ie its illegal to drive without car insurance, in the 30th century ship systems could stop ships from being flown without valid insurance), and it'd almost certainly spam you with warnings or something to that effect. It shouldnt be a concern, in terms of 'can i lose my ships' or whether you're unfairly disadvantaged not having LTI.
The locking & stealing, current state its completely meaningless really. Theres no locks, and if someone steals your ship its still yours to spawn back in. If you leave the game, shortly after the ship is despawned (leaving the thief stranded in space).
In the PU, once things are fully implemented, locking will definitely be a thing, and people will be able to board your ship in many ways, and attempt to steal it. In doing so, the ships Hull ID will be marked stolen, and anyone flying it will be popular with UEE patrols. Whether you can strip it down for parts, blow it up and salvage its scraps, is unknown really, theres also been suggestions that you'd be able to take the ship to a less reputable planet and have forged documents and whatnot to keep it. The owner of the stolen ship would just get a replacement from insurance.
Nothing will be catastrophic really, there are real-world systems to add realism, and make the illegal lifestyles some people will live, viable and challenging, without it being particularly damaging to the people playing a law-abiding life.
Thats part of whats always drawn me to SC, they're attention to detail and building a realistic universe to play in, but its designed to be fair and fun, while still addressing real matters.
Reading
Death of a Spaceman is all it takes, IMO.